wc: improve avx2 API

wc: improve avx2 API

* src/wc.c: Use "#include <...>" for files not in the current dir.
Include "wc.h" instead of declaring wc_lines_avx2 by hand.
(wc_lines): New API, with no file name (no longer needed) and
with a return struct rather than arg pointers.  All uses changed.
Use avx2_supported directly instead of using a function pointer.
Exploit C99-style declarations after statements.
Multiply by 15 rather than dividing; it’s faster and more accurate
and cannot overflow here.
(wc): Simplify based on wc_lines API change.
* src/wc.h: New file.
* src/wc_avx2.c: Include it, to check API better.
(wc_lines_avx2): Use new API.  All uses changed.  Exploit C99.
Make locals more local.