Remove `bump_invisible()` where possible

[c42f]

Looking at the use of bump_invisible(), it's used for some useful things:

• Emitting errors when expected syntax isn't present
• Emitting empty tokens
  • var""
  • preparing triple string trivia (avoiding backtracking)

And some dubious things

Premature lowering (see also #88)

• juxtuposition lowering to call in the parser
• return becoming return nothing implicitly
• lowering core macro calls early
  • core_@doc
  • core_@cmd
• empty strings have one chunk

Dealing with the fact that Expr heads don't have flags

• mutable struct vs struct use a bool flag
• module vs baremodule use a bool flag
• try uses bool flags for its optional sub-clauses (see Reworking some Julia AST forms #88)

We should probably remove the dubious usages.

[c42f]

I think this one is as done as it's going to be.