Add type restrictions to Array#concat

[yui-knk]

To stop something

in ./src/array.cr:420: no overload matches 'Pointer(Int32)#value=' with types (String | Int32 | Char)
Overloads are:
 - Pointer(Int32)#value=(value : Int32)
Couldn't find overloads for these types:
 - Pointer(Int32)#value=(value : String)
 - Pointer(Int32)#value=(value : Char)

      buf.value = elem
          ^~~~~

but I still face these bug

a = [1, 2, 3]
a.concat(["4", 'e', 2])

# compile error
Using compiled compiler at .build/crystal
Error in crystal/one.cr:24: instantiating 'Array(Int32)#concat(Array((String | Int32 | Char)))'

      a.concat(["4", 'e', 2])
        ^~~~~~

in ./src/array.cr:414: instantiating 'Array((String | Int32 | Char))#each()'

    other.each do |elem|
          ^~~~

in ./src/array.cr:497: instantiating 'each_index()'

    each_index do |i|
    ^~~~~~~~~~

in ./src/array.cr:497: instantiating 'each_index()'

    each_index do |i|
    ^~~~~~~~~~

in ./src/array.cr:414: instantiating 'Array((String | Int32 | Char))#each()'

    other.each do |elem|
          ^~~~

in ./src/array.cr:420: no overload matches 'Pointer(Int32)#value=' with types (String | Int32 | Char)
Overloads are:
 - Pointer(Int32)#value=(value : Int32)
Couldn't find overloads for these types:
 - Pointer(Int32)#value=(value : String)
 - Pointer(Int32)#value=(value : Char)

      buf.value = elem
          ^~~~~

I can not find out why concat(other : Enumerable(T)) is called, sorry :(

[asterite]

@yui-knk I think that it enters Enumerable is a bug. I'll try to fix it before merging this. Thanks!

[asterite]

It turns out this depends on resolving #1297 first.

[asterite]

I'll close this for now. The compiler still rejects concatenating an array with non-compatible types. The error could definitely be improved, but until we define how we want variance to work I'll close this as we won't tackle it soon.