fix Issue 10403 - memchr optimization for std.algorithm.find

Basically, optimizations for find(R, E).

Contains:
- Regardless of pred:
  - Optimization for raw arrays (no speedup with inline+release, but MAJOR speed up in both non release or non inline, so worth it (IMO)).
  - Removed "default "a == b"" pred
  - Found the case where pred == "a == b", and changed it to lambda, (for when called by other algos).
    - Doing this also avoids pass by value with binaryFun.

- No pred or "a == b"
  - Optimization for finding 1Byte POD in a 1Byte POD sequence.
  - Generic optimization for narrow strings.
  - even better optimization for narrow strings when the needle fits in a single codeunit.

Unittests.
Made sure it works in safe, pure or ctfe context.
Fixes in find(R, E)