Possibility of a "global time source" for wasm targets

What we need is not another special case -- libs-api has talked about not wanting to own more of these -- but some generalized language feature for pluggable implementations for things that allocators could also migrate to.

Imagine some kind of #[magic_pluggable_thing] pub static ALLOCATOR: impl GlobalAlloc; that different crates could provide, say.

We'd then use that for all kinds of things -- float parsing, for example, which is giant in code size so people might want to just always-panic or use a shared implementation between their rust and C code rather than the one in core.

(That'll take a bunch of design work, though, across lots of stuff.) Oh, see below.