Optimization & factorization of ‘write’

[Ludovic Courtès]

Hi!

In a profile ‘write’ was showing up unreasonably high, so I decided to
bite the bullet, so to speak.

I ended up factorizing character and string writing, and removing heap
allocations from the path.  The end result is ~2.6x faster when writing
strings.

Feedback welcome, especially from Mike.  :-)

Thanks,
Ludo’.