feat: improve performance of C implementation

[akx]

This PR:

• improves the performance of the C implementation by eschewing C++ std::string allocations in favor of simple byte buffers and trying to make sure the Cython code does as little copying and unnecessary non-ASCII decoding as possible.
• lifts the "get timestamp" function from the tests to a public API and adds a C implementation for it.

---

Caution

I tried to be extra fastidious and careful about the use of raw buffers here, but it's likely a good idea to look over this with a pretty fine comb.

---

On my machine, this seems to improve performance approximately thus:

• ulid_now_bytes: 1,439 OPS -> 1,757 OPS (+22%)
• ulid_hex: 1,057 OPS -> 1,414 OPS (+33%)
• ut_decode: 8,862 OPS -> 12,660 OPS (+42%)
• ulid_at_time: 5,879 OPS -> 6,390 OPS (+9%)
• ulid_now: 5,972 -> 6,189 OPS (+3%)

The new timestamp parsing API is also pretty snappy:

--------------------- benchmark 'timestamp': 4 tests ---------------------
Name (time in ns)                   Mean            OPS (Kops/s)
--------------------------------------------------------------------------
test_ut_timestamp[c]             81.4350 (1.0)       12,279.7339 (1.0)
test_ut_timestamp[python]     1,855.7862 (22.79)        538.8552 (0.04)
test_ulidpy_uuid              2,843.5127 (34.92)        351.6777 (0.03)
test_ulid2_timestamp          3,261.0542 (40.04)        306.6493 (0.02)
--------------------------------------------------------------------------

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (975079d) to head (d7c1d00).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #43      +/-   ##
===========================================
+ Coverage   95.74%   100.00%   +4.25%     
===========================================
  Files           2         2              
  Lines          47        52       +5     
  Branches        4         5       +1     
===========================================
+ Hits           45        52       +7     
+ Misses          2         0       -2     

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[bdraco]

Buffer sizes all look right.

[bdraco]

generated code looks good

cythonize -X language_level=3 -a -i src/ulid_transform/_ulid_impl.pyx

[bdraco]

[bdraco]

pushed on production system.
all good

[bdraco]

Thanks @akx
Nice work!