Improve the encoding scheme of vint

[tomtomwombat]

PR has two changes:

1. When comparing two heap ColdStrings, we can read and compare the first WIDTH bytes without parsing the vint. This is a fast path and works because strings are only stored on heap iff there are more than WIDTH bytes. We don't read WIDTH + 2 bytes, even though we could (accounting for at least 1 vint byte, plus WIDTH + 1 or more UTF-8 bytes) because it's slower (probably because it requires more reads?).
2. Optimize the vint encoding scheme. I tried a bunch, but the one here is the fastest, and compact too. It's adapted from https://www.inkandswitch.com/tangents/bijou64/ with cold-string specific optimizations, because we can always read WIDTH + 2 bytes from heap, and mask away junk UTF-8 bytes past the var-int's len.

Benchmark: as_str (ns/op)

Type	len=0..=4	len=0..=8	len=0..=16	len=0..=32	len=0..=255	len=4..=4	len=8..=8	len=16..=16	len=32..=32	len=255..=255
String	0.69	0.69	0.69	0.69	0.69	0.69	0.69	0.69	0.69	0.69
cold_string::ColdString	1.66	1.61	1.20	1.00	0.94	1.62	1.63	0.77	0.80	0.83
compact_str::CompactString	0.75	0.76	0.75	0.72	0.74	0.76	0.74	0.75	0.71	0.70
compact_string::CompactString	0.73	0.72	0.72	0.73	0.76	0.72	0.72	0.72	0.75	0.76
smallstr::SmallString<[u8; 8]>	0.67	0.67	0.85	0.69	0.73	0.67	0.68	0.67	0.66	0.66
smartstring::alias::String	2.43	2.43	2.43	2.32	1.49	2.43	2.43	2.43	1.33	1.34
smol_str::SmolStr	0.67	0.67	0.67	0.66	0.81	0.67	0.68	0.67	0.68	0.67

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Benchmark: eq (ns/op)

Type	len=4, eq=0	len=4, eq=0.5	len=4, eq=1	len=8, eq=0	len=8, eq=0.5	len=8, eq=1	len=16, eq=0	len=16, eq=0.5	len=16, eq=1	len=32, eq=0	len=32, eq=0.5	len=32, eq=1	len=255, eq=0	len=255, eq=0.5	len=255, eq=1
String	2.33	2.98	2.74	2.51	2.94	2.60	2.39	2.97	2.81	2.20	2.84	2.62	2.18	11.61	13.55
cold_string::ColdString	1.31	1.31	1.31	1.31	1.31	1.31	1.42	4.79	5.72	1.44	5.73	6.68	1.53	12.25	15.54
compact_str::CompactString	2.90	3.95	3.24	3.04	3.85	3.25	3.04	4.44	3.47	2.84	3.55	3.30	2.84	12.80	14.56
compact_string::CompactString	2.45	3.03	2.83	2.05	3.17	3.96	2.06	4.21	4.53	2.03	5.02	5.11	2.11	12.82	14.62
smallstr::SmallString<[u8; 8]>	2.45	3.25	3.03	2.81	3.03	2.81	3.04	3.87	3.47	2.83	4.56	3.26	2.87	11.99	13.97
smartstring::alias::String	6.55	7.42	6.89	6.13	7.33	7.38	6.11	9.05	7.99	4.97	6.40	5.42	4.97	13.69	14.18
smol_str::SmolStr	4.21	3.29	1.96	3.77	3.13	1.96	3.84	3.33	2.08	4.33	5.72	4.59	4.19	13.54	15.20

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Benchmark: len (ns/op)

Type	len=4	len=8	len=16	len=255
String	0.43	0.43	0.43	0.43
cold_string::ColdString	0.42	0.43	0.43	0.43
compact_str::CompactString	0.42	0.42	0.43	0.43
compact_string::CompactString	0.43	0.43	0.43	0.43
smallstr::SmallString<[u8; 8]>	0.43	0.42	0.42	0.43
smartstring::alias::String	1.29	1.28	1.30	1.29
smol_str::SmolStr	0.43	0.42	0.45	0.45

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Benchmark: new (ns/op)

Type	len=0..=4	len=0..=8	len=0..=16	len=0..=32	len=0..=255	len=4..=4	len=8..=8	len=16..=16	len=32..=32	len=255..=255
String	29.64	31.99	35.71	33.84	41.24	28.58	28.80	29.55	30.34	33.59
cold_string::ColdString	10.77	10.22	25.68	30.44	43.24	6.25	5.04	33.79	33.74	37.48
compact_str::CompactString	11.52	12.06	12.31	18.07	53.39	10.88	10.84	10.95	38.46	45.86
compact_string::CompactString	34.95	36.84	40.28	38.24	44.93	29.88	30.26	31.84	32.20	35.72
smallstr::SmallString<[u8; 8]>	9.52	10.01	22.50	29.03	41.24	4.81	2.38	29.06	31.15	33.29
smartstring::alias::String	15.60	15.82	15.66	31.46	88.72	15.67	15.65	10.89	160.37	72.01
smol_str::SmolStr	17.61	17.93	15.99	22.94	55.73	18.27	13.86	13.75	39.46	42.07

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