[InAnotherWorld] Use @locals to determine the variables inscope at a breakpoint

[oxinabox]

The last commit is where this diverges from #48 it would be a massive simplication
if only it worked.

Unfortunately, it does not as: It is not permitted to use Expr(:locals) in a Cassette pass. Because that gets expanded before/during the creation of the lowered IR (edited) Looks like it gets expend in https://github.com/JuliaLang/julia/blob/553082495c46e851db1d287c917963b408e0db6e/src/julia-syntax.scm#L2433

It is basically expanded into the code that I wanted to replace with it.

julia> function f(x)
       local y
       println(Base.@locals())
       end
f (generic function with 1 method)

julia> @code_lowered f(1)
CodeInfo(
1 ─       Core.NewvarNode(:(y))
│   %2  = (Core.apply_type)(Base.Dict, Core.Symbol, Core.Any)
│   %3  = (%2)()
│   %4  = $(Expr(:isdefined, :(y)))
└──       goto #3 if not %4
2 ─       (Base.setindex!)(%3, y, :y)
3 ┄ %7  = $(Expr(:isdefined, :(x)))
└──       goto #5 if not %7
4 ─       (Base.setindex!)(%3, x, :x)
5 ┄ %10 = (Main.println)(%3)
└──       return %10
)