PHP-962: Define constants for MongoBinData subtypes 0 and 4

Subtype 0 is the new default (to be changed in PHP-407), which replaces deprecated subtype 2. Subtype 4 is the new UUID type, which enforces the RFC spec, to replace deprecated subtype 3.

Additionally, replace hard-coded values in bin_data.c with the defined constants.
Merge pull request #571