Improve JSON str encode performance

[jcrist]

This improves the performance of encoding JSON strings, mostly by rearranging some work and unrolling some loops.

[jcrist]

A quick benchmark of string performance:

import random
import string
from time import perf_counter_ns

from orjson import dumps
from msgspec.json import encode

CHARS = string.ascii_letters
CHARS_WITH_ESCAPES = "\x01\n'\"" + string.ascii_letters


def randstr(chars):
    k = random.randint(2, 64)
    return "".join(random.choices(chars, k=k))


N = 1000
M = 10000
random.seed(42)

for header, chars in [("No Escapes:", CHARS), ("With Escapes:", CHARS_WITH_ESCAPES)]:
    print(header)
    data = [randstr(chars) for _ in range(N)]

    start = perf_counter_ns()
    for _ in range(M):
        encode(data)
    stop = perf_counter_ns()
    print(f"- msgspec: {(stop - start) / (1000 * M):.1f} us")

    start = perf_counter_ns()
    for _ in range(M):
        dumps(data)
    stop = perf_counter_ns()
    print(f"- orjson: {(stop - start) / (1000 * M):.1f} us")

Before

$ python bench.py
No Escapes:
- msgspec: 29.7 us
- orjson: 21.5 us
With Escapes:
- msgspec: 53.1 us
- orjson: 67.0 us

This PR

$ python bench.py
No Escapes:
- msgspec: 18.6 us
- orjson: 22.0 us
With Escapes:
- msgspec: 34.2 us
- orjson: 68.1 us

Notes:

• This PR makes msgspec ~1.6x faster for encoding all strings
• orjson is optimized towards the common case of encoding strings that don't have characters needing escaping (\n, \t, \r, \f, \b, ", ', \, and anything in \x01 - \x32). In the case of strings where escape codes are needed, orjson's performance tanks. msgspec was already faster than orjson for encoding strings that require escape codes, now we're ~2x faster in that case. We're also now faster in the common case.

Even though this PR unrolls some loops, it actually results in a (negligible) decrease in binary size since we delete a non-inlined helper function for handling the escape code case.