Make ryu an optional dependency

[DenialAdams]

You should be able to import "ryu", instead of pulling it in automatically just by using STD. Basically, this issue is about a better way to manage standard library imports

[DenialAdams]

Some progress made here, but f32_to_string requires malloc, so we can't put it in the ryu module directly, so it has to stay a direct dependency of the WASI std... Hm

[DenialAdams]

need something like

when(defined(__ryu)) { proc f32_to_string { } }

?