Keeping track of line numbers

[timholy]

In JuliaCI/Coverage.jl#36, I'm trying to leverage this to improve our accounting for code coverage. The biggest challenge is keeping track of line numbers appropriately. Here's a good example of the challenge (save this to a file /tmp/testparser.jl):

f1(x) = 2x
if isdefined(:f1)
    f2(x) = 3x
end
f3(x) = 4x

Here's what comes from parsing this file:

julia> io = open("/tmp/testparser.jl")
IOStream(<file /tmp/testparser.jl>)

julia> ast = Parser.parse(io)
:(f1(x) = begin  # none, line 1:   # we're at line 1
            2x
        end)

julia> ast = Parser.parse(io)
:(if isdefined(:f1) # line 2:
        f2(x) = begin  # none, line 2:   # we're at line 2+1 = 3
                3x
            end
    end)

julia> ast = Parser.parse(io)
:(f3(x) = begin  # none, line 1:    # we're at line 1+3 = 4?
            4x
        end)

Because the end doesn't get its own line number, it doesn't seem possible to keep track of the line number just by looking for :line exprs or LineNumberNodes.

[timholy]

Much less important now that I used a hack in Coverage.jl](https://github.com/timholy/Coverage.jl/commit/c244d43df3c393e877a40cc2dea395be5a2de59f). In several respects this hack is probably the right way to do it; there's no bookkeeping of any kind, so no risk you'll ever be wrong.

I'll let you decide what you want to do with this issue.

[jakebolewski]

I want to preserve compatibility with the scheme parser. We could have the parse function take a line parameter to initialize the parse state, the reported line number would be offset from this based on the input stream. I think this is more of a limitation of how we do line numbers in general for Julia.