This adds a new `str_values` boolean keyword argument to
`from_builtins`, defaulting to `False`. If set to `True`, `int`,
`float`, `bool`, and `None` types will also support decoding from `str`
values. This is useful for protocols that only have string scalars (like
URL query parameters).

Ordered by type likelihood in common message structures.

A micro-optimization, saves ~600 ns per call on my machine. If this
wasn't easy to do it wouldn't be worth it.

Adds prose docs for `from_builtins` and `to_builtins`.

jcrist deleted the from-builtins-str-values branch