mapexpr argument for include: transform each parsed expression

[stevengj]

This PR adds an optional mapexpr keyword first argument to include and include_string, allowing you to specify a function that rewrites each expression after it is parsed. I've needed this functionality several times, e.g. in SoftScope.jl and ChangePrecision.jl, and it is tricky implement manually in a way that preserves line-number information and other semantics of include.

Moreover, now that #33864 has merged, I would like to use the "official" REPL.softscope! function in IJulia instead of SoftScope.jl, or equivalently change SoftScope.jl to use this function on newer Julia releases. With this PR, one can just pass REPL.softscope! to include_string.

To do:

• Tests
• Documentation
• Benchmarking. Invoking the identity transformation x->x adds a 2–3% overhead to include_string measured for "nothing\n"^10000, but I eliminated this by optimizing the default case of mapexpr === identity.

[stevengj]

@JeffBezanson, where is a module's include(x) function defined? I need to add an include(mapexpr, ex) method there… Update: found it in jlfrontend.scm, see below.

[StefanKarpinski]

I like the mapping function as the first argument. What else would one pass there, after all?

[stevengj]

If I do

julia> module Foo; end; methods(Foo.include)
# 1 method for generic function "include":
[1] include(x) in Main.Foo at REPL[1]:1

where is Foo.include(x) defined? I'm having trouble tracking it down.

Update: I found it — it's in module-default-defs in jlfrontend.scm.

[stevengj]

Rebased. Is there anything else that is needed here?

[StefanKarpinski]

I think only for @JeffBezanson to have a look. He was traveling last week so a bit occupied. He'll probably be able to take a look and merge if he approves this week.