More robust inf/nan recognition and generation.

More robust inf/nan recognition and generation.

Drop INFNAN_PEEK, premature optimization and hard to get right (it
basically imitates unrolled first half of grok_infnan).  Just keep
grok_infan fast.  (There is one spot in grok_number_flags() where we
peek at the next byte to avoid wasted work.)

If falling back (from not having NV_INF/NV_NAN) to the native strtod
(or similar), fake the input based on the grok_infnan result.
Add last-resort ways to generate inf/nan.

Recognize explicit unary plus, like "+Inf", and "INFINITE".

In tests use cmp_ok(), fix typos, add tests.