Add global string de-duplication cache

[tiif]

Currently, in localtime_r and also for some cases where we raise panics, we allocate a fresh string each time. These strings never get deallocated so they just keep accumulating. Conceptually these are static strings so we can deduplicate them with a global cache.

The cache should be used everywhere that allocate_str is called with MiriMemoryKind::Machine.

[RalfJung]

(I have updated the issue title and description.)

[dev-ardi]

There exist many string interning crates, are you leaning towards any specific one?

[RalfJung]

None of them are usable I think, since the strings need to be interned in machine memory.

This should be just a FxHashMap<Vec<u8>, Pointer<Option<Provenance>>>.