Add each_hinted_uniform method.

Iterates over all uniforms previously hinted to exist (regardless of
whether they do). Uniforms that don't exist are not currently removed,
but this might change in the future. Don't depend on it staying around.
Fix uniform location caching issue with hint_uniform.

hint_uniform set a uniform's cached location to -1, causing it to fail
to reload. This is mainly because __reload_uniforms__ used
uniform_location to get a new uniform location, but that doesn't really
work out because uniform_location only returns cached values (or gets a
uniform's location, if not already cached, and caches it).
Version 1.3.2

- Remove Program#kind
- Add Program#each_hinted_uniform
- Fix uniform location caching issue from Program#hint_uniform use and
  __reload_uniforms__ stupidity.