Avoid two statics for every callsite

[Mark-Simulacrum]

Enhancement

Motivation

Currently tracing generates two statics: META and CALLSITE for every callsite. It seems likely that this has a higher cost at codegen and (potentially) runtime than just one static, as it's adding to the amount of symbols in the binary, etc.

That said, we don't have actual measurements that say "this is concretely bad" :)

Proposal

Probably the easiest thing is to try and refactor the code such that the macro-generated static is just static F: (&Metadata<'static>, Callsite) = (..., ...); -- but maybe there's better alternatives than that.