std: change XXX to NOTE
std: Remove XXX, this doesn't seem to need fixing
std: get rid of awkward Either thing

Eliminates an allocation, and an XXX
std: Get rid of by-value-self workarounds; it works now
std: Remove type annotations, they're not needed anymore
std: Comments
std/extra: Move http module to extra
std: XXX to FIXME (comments only)
std: avoid allocations in sys
std: Comments only
auto merge of #7812 : catamorphism/rust/std-cleanup, r=catamorphism

Mostly changing them to numbered FIXMEs. Inoffensive changes, self-reviewing.