## Summary

This introduces a new workspace member, `uv-fastid`. `uv-fastid`
provides IDs using the same alphabet as nanoid, but with a slightly
smaller token size (16 instead of 21) and fewer knobs for configuration.

In practice it should also be faster, since our IDs are now trivially
copyable + don't require any heap allocations to construct internally.
This is probably not a huge deal in our case anyways (ID generation
isn't exactly dominating our runtime), but it doesn't hurt to have a
fast primitive here.

The two main APIs I've added are `insecure()` and `secure()`, whose
names refer to the kind of PRNG they use internally. Right now we only
use `insecure()`, since we don't actually need a CSPRNG for the uses of
nanoid we're replacing (revision and archive IDs).

Some other notes:

- Arguably we could remove the secure/insecure distinction entirely, and
just keep the CSPRNG path -- with `rand` we have a purely userspace
CSPRNG (seeded from the OS's CSPRNG), so we don't really need `fastrand`
for performance here.
- I'm guilty of using `unsafe` in this, although I think the usage is
demonstrably safe (as long as crate-private invariants are maintained) 🙂
- I think there's no breakage risk with this -- FWICT our only usage of
`nanoid` doesn't make any format assumptions, so switching to a slightly
shorter ID form here shouldn't break anything. Our previous IDs also
aren't part of any public interface AFAICT.

Closes #19176.

## Test Plan

Added unit tests to the new `uv-fastid` crate. Our existing tests should
cover this transitively as well.

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Signed-off-by: William Woodruff <william@astral.sh>