Make static_show() print valid identifiers

[NHDaly]

This PR makes a few separate changes to static_show(), to bring it more in line with julia's show(), and to fix cases where it was printing non-valid code.

• Use var"" syntax in static_show to print names containing invalid characters as valid identifiers.
• Start printing function instances by instance name, instead of via a non-existent type constructor.

Commit messages follow:

---

Use var"" syntax in static_show to print valid identifiers

Previously, static_show was printing invalid identifiers for
TypeVars that contain a #, such as those produced by gensym().

For example:

julia> println(static_shown(Vector{<:Bool}))
Array{#s5, 1} where #s5<:Bool

julia> eval(Meta.parse(static_shown(Vector{<:Bool})))
ERROR: syntax: incomplete: premature end of input

After this commit, this is printed as a valid identifier:

julia> println(static_shown(Vector{<:Bool}))
Array{var"#s5", 1} where var"#s5"<:Bool

julia> eval(Meta.parse(static_shown(Vector{<:Bool})))
Vector{var"#s5"} where var"#s5"<:Bool (alias for Array{var"#s5", 1} where var"#s5"<:Bool)

As you can see, this mirrors the printing done from julia's show().

This commit also applies the same technique to simplify the printing for
Type names that contain a #, simply for asthetic reasons and
consistency with julia show(), by reusing the above functionality.

Before:

julia> struct var"#X#" x::Int end

julia> println(static_shown(var"#X#"))
getfield(Main, Symbol("#X#"))

julia> println(static_shown(var"#X#"(0)))
getfield(Main, Symbol("#X#"))(x=0)

After:

julia> struct var"#X#" x::Int end

julia> println(static_shown(var"#X#"))
Main.var"#X#"

julia> println(static_shown(var"#X#"(0)))
Main.var"#X#"(x=0)

---

Make functions print as valid julia expressions.

Previously, function instances would be printed in static_show() as
elements of a DataType, with no special printing for <:Functions.
This led to printing them via invalid syntax, since there is no empty
constructor for T<:Function.

Before:

julia> g() = 2
g (generic function with 1 method)

julia> println(static_shown(g))
typeof(Main.g)()

julia> eval(Meta.parse(static_shown(g)))
ERROR: MethodError: no method matching typeof(g)()

After:

julia> g() = 2
g (generic function with 1 method)

julia> println(static_shown(g))
Main.g

julia> eval(Meta.parse(static_shown(g)))
g (generic function with 1 method)

I chose to keep the Module prefix on the functions, so that they will
be valid julia expressions that you can enter at the REPL to reproduce
the function instance, even though this is a break from the julia
show() behavior.

One caveat is that this is still not correct for anonymous functions,
but I'm also not really sure what would be a better way to print
anonymous functions... I think this is probably better than it was
before, but very open to suggestions for how to improve it.

Before:

julia> l = (x,y)->x+y

julia> println(static_shown(l))
getfield(Main, Symbol("#3#4"))()

julia> eval(Meta.parse(static_shown(l)))
ERROR: MethodError: no method matching var"#3#4"()

After:

julia> l = (x,y)->x+y

julia> println(static_shown(l))
Main.var"#3"

julia> eval(Meta.parse(static_shown(l)))
ERROR: UndefVarError: #3 not defined

---

Extend static_show() escaping to any illegal identifier via var"".

Previously, it was only escaping names with a # in them, but now it
will escape all names that are illegal identifiers.

Before:

julia> struct var"a%b+c" x::Int end

julia> println(static_shown(var"a%b+c"))
Main.a%b+c

julia> println(static_shown(var"a%b+c"(1)))
Main.a%b+c(x=1)

After:

julia> struct var"a%b+c" x::Int end

julia> println(static_shown(var"a%b+c"))
Main.var"a%b+c"

julia> println(static_shown(var"a%b+c"(1)))
Main.var"a%b+c"(x=1)

[NHDaly]

This addresses Case 2 in this issue about emitting invalid precompile statements: #28808

[Sacha0]

First commit looks great! :)

[Sacha0]

Second commit looks great! --- modulo a couple things I don't entirely grok; let's chat offline :).

[Sacha0]

Third commit looks great! :)

[Sacha0]

Confusion resolved. Looks great! :)

[Sacha0]

Absent objections or requests for time, I plan to merge this pull request on Saturday morning PT. Best! :)

[JeffBezanson]

Is the isoperator check really needed? For example this gives typeof(Base.:(var"+")) instead of typeof(Base.:(+)).

[NHDaly]

Oops! thanks Jeff. I really think i meant to make this if (!(jl_is_identifier(sn) || jl_is_operator(sn))). Because already jl_is_identifier("+") returns false, so i think i added this specifically to allow operators to be okay, but i screwed up the parens. That's what i get for coding late at night 😅 thanks - this is a great catch!

julia> @ccall jl_is_identifier("+"::Cstring)::Bool
false

julia> @ccall(jl_is_identifier("+"::Cstring)::Bool) || @ccall(jl_is_operator("+"::Cstring)::Bool)
true

i've amended the commit to fix this, and also added an expected output test for this case. 👍 thanks!

[NHDaly]

This is now fixed. Does this sound right to you? Thanks.