Optimization of ‘peek-char’

[Ludovic Courtès]

Howdy,

Commit fd5eec2b6e113f6d13028215a738417607432a2d optimizes ‘peek-char’,
the main motivation being that SSAX uses it a lot[*].

It makes ‘peek-char’ 40x faster on a port whose encoding is #f, 1.9x
faster on a UTF-8 port containing unibyte codepoints, and 1.3x faster on
a UTF-8 port containing multi-byte codepoints (measured using
‘write.bm’).

The ‘xml->sxml’ procedure is 4x faster on the 2.7 MiB XML file that I
used for testing.

There’s still room for optimization, though...

Thanks,
Ludo’.

[*] I just noticed that one line is missing from the commit log.
    Perhaps I should add a git note to fix it?