[ty] Add environment variable to dump Salsa memory usage stats (#18928)

## Summary

Setting `TY_MEMORY_REPORT=full` will generate and print a memory usage
report to the CLI after a `ty check` run:

```
=======SALSA STRUCTS=======
`Definition`                                       metadata=7.24MB   fields=17.38MB  count=181062
`Expression`                                       metadata=4.45MB   fields=5.94MB   count=92804
`member_lookup_with_policy_::interned_arguments`   metadata=1.97MB   fields=2.25MB   count=35176
...
=======SALSA QUERIES=======
`File -> ty_python_semantic::semantic_index::SemanticIndex`
    metadata=11.46MB  fields=88.86MB  count=1638
`Definition -> ty_python_semantic::types::infer::TypeInference`
    metadata=24.52MB  fields=86.68MB  count=146018
`File -> ruff_db::parsed::ParsedModule`
    metadata=0.12MB   fields=69.06MB  count=1642
...
=======SALSA SUMMARY=======
TOTAL MEMORY USAGE: 577.61MB
    struct metadata = 29.00MB
    struct fields = 35.68MB
    memo metadata = 103.87MB
    memo fields = 409.06MB
```

Eventually, we should integrate these numbers into CI in some form. The
one limitation currently is that heap allocations in salsa structs (e.g.
interned values) are not tracked, but memoized values should have full
coverage. We may also want a peak memory usage counter (that accounts
for non-salsa memory), but that is relatively simple to profile manually
(e.g. `time -v ty check`) and would require a compile-time option to
avoid runtime overhead.

crates/ty_python_semantic/src/util/get_size.rs

@@ -0,0 +1,15 @@
+use std::sync::Arc;
+
+use get_size2::GetSize;
+
+/// By default, `Arc<T>: GetSize` requires `T: 'static` to enable tracking references
+/// of the `Arc` and avoid double-counting. This method opts out of that behavior and
+/// removes the `'static` requirement.
+///
+/// This method will just return the heap-size of the inner `T`.
+pub(crate) fn untracked_arc_size<T>(arc: &Arc<T>) -> usize
+where
+    T: GetSize,
+{
+    T::get_heap_size(&**arc)
+}

crates/ty_python_semantic/src/util/mod.rs

@@ -1,2 +1,3 @@
 pub(crate) mod diagnostics;
+pub(crate) mod get_size;
 pub(crate) mod subscript;