Motivation:
Very unfortunately [SR-7602](https://bugs.swift.org/browse/SR-7602)
Swift's String doesn't work very well on UTF-8 encoded data. Fortunately
(but unfairly see point (3) in
[SR-7602](https://bugs.swift.org/browse/SR-7602)) however there is now a
contiguous String storage for ASCII-only Strings that _only the stdlib_
can access. To benefit from the stdlib's capabilities of enumerating the
bytes faster than we can, we should use UnsafeMutableBufferPointer#initialize(from: Bytes).
Thanks @airspeedswift for pointing this out.
The performance gains on a recent Swift version are noticeable:
after this patch:
```
no-net_http1_10k_reqs_1_conn: 0.225782990455627, 0.228317022323608, 0.225206971168518, 0.22549307346344, 0.223728060722351, 0.225827932357788, 0.222437977790833, 0.221745014190674, 0.220004916191101, 0.222965002059937
http1_10k_reqs_1_conn: 0.688782095909119, 0.682757973670959, 0.689703941345215, 0.708670020103455, 0.691922068595886, 0.687381982803345, 0.681567072868347, 0.684866905212402, 0.683283090591431, 0.699701905250549
http1_10k_reqs_100_conns: 0.782313942909241, 0.779283046722412, 0.773630023002625, 0.776782989501953, 0.774596929550171, 0.775121927261353, 0.773631930351257, 0.784304022789001, 0.795854926109314, 0.786051988601685
```
before this patch:
```
no-net_http1_10k_reqs_1_conn: 0.232797980308533, 0.236240983009338, 0.237086057662964, 0.236070990562439, 0.236560940742493, 0.236254930496216, 0.234392046928406, 0.235393047332764, 0.234088897705078, 0.245710015296936
http1_10k_reqs_1_conn: 0.76228404045105, 0.788464903831482, 0.747326016426086, 0.753327012062073, 0.735787987709045, 0.727090001106262, 0.720335006713867, 0.721843004226685, 0.719871044158936, 0.728644967079163
http1_10k_reqs_100_conns: 0.841314911842346, 0.884468078613281, 0.854345917701721, 0.847344040870667, 0.847607016563416, 0.841101050376892, 0.814589977264404, 0.818693995475769, 0.832610964775085, 0.828818082809448
```
Modifications:
use UnsafeMutableBufferPointer#initialize(from: Bytes) to enumerate
String's bytes.
Result:
faster but still not fast enough, we need SR-7602 to be addressed
a7ed5fd4e9