Have CanonicalizePath handle \ on Windows

Have CanonicalizePath handle \ on Windows. It's slightly slower to do so, so compensate for that by improving ReadFile to not use text mode (which is a bit slower), and then fixing the lexer to accept \r (it was previously relying on reading files in text mode to remove '\r's).

Timings:

before patch:
qpc: 1229494us
qpc: 1232420us
qpc: 1232493us
qpc: 1235035us
qpc: 1239521us
qpc: 1247100us
avg = 1234867us

with canon of \, fixing parser to accept \r and speedier ReadFile
qpc: 1196304us
qpc: 1200643us
qpc: 1187478us
qpc: 1190062us
qpc: 1182991us
qpc: 1203037us
avg = 1190128us

Shouldn't this normalize to `x/bar.h`?