Improve Performance of HTTPField.value

[nerdsupremacist]

Motivation

When instantiating new HTTPFields, the values need to be validated, legalized and turned into the internal representation.
We use this representation to expose a string in HTTPField.value as well as to iterate over raw bytes using HTTPField.withUnsafeBytesOfValue.

Currently the internal representation is to use a ISO Latin 1 String to store the bytes only once. This however means that we need to transcode back and forth for non ascii values, which causes a lot of extra computations and allocations.

Modifications

1. Moved logic to a standalone structure for HTTPField.Value rather than ISOLatin1String
2. Added Unit Tests and Benchmarks
3. Swapped internal storage of values to be an enum of either a string or a value
4. Whenever the bytes stored are valid utf8, we will not always store the value as a raw string. This means that we need the least amount of allocations and transcoding when operating with the value through our public API
5. We only store a raw array of bytes whenever the bytes are not valid utf8. This speeds up withUnsafeBytesOfValue massively and HTTPField.value a little bit
6. During legalization, store the modified bytes in a temporary buffer instead of using lazy collections to avoid allocating an array and speed up utf8 validation
7. Use spans whenever possible to speed up access to bytes without boundary checks

Result

These are the changes on the benchmarks added in this PR

HTTPField-asciiBytesValueRoundTrip
Instructions: 28K -> 6847 (-75%)
Malloc: 1 -> 1

HTTPField-asciiValueRoundTrip
Instructions: 7367 -> 3081 (-58%)
Malloc: 0 -> 0

HTTPField-invalidAsciiBytesValueRoundTrip
Instructions: 133K -> 7255 (-94%)
Malloc: 6 -> 1 (-83%)

HTTPField-invalidAsciiBytesValueRoundTripAsLenientValue
Instructions: 62K -> 6623 (-89%)
Malloc: 3 -> 1 (-66%)

HTTPField-invalidAsciiValueRoundTrip
Instructions: 93K -> 6959 (-92%)
Malloc: 5 -> 1 (-80%)

HTTPField-invalidNonASCIIValueRoundTrip
Instructions: 393K -> 8735 (-97%)
Malloc: 11 -> 1 (-91%)

HTTPField-nonASCIIValueRoundTrip
Instructions: 131K -> 3219 (-97%)
Malloc: 4 -> 0 (-100%)

[fabianfett]

Thank you!