lsi is a leaking string interner. In return for leaking, you get ludicrous speed. The "l" stands
for either "leaking" or "ludicrous", depending on your mood.
Use it when you'll quickly hit saturation on the number of distinct strings you'll need to store: compilers, http stacks, etc.
Istr is usize-sized, and points directly to string data on the heap, where the layout is [len, ...data].
This means that Istr is Copy and single-instruction equality checks.
When constructing a Istr, we use a global hashbrown::HashSet to see if the string already exists. hashbrown uses ahash,
which benchmarks show as the fastest mainstream str hasher.