Parse Timestamp argument as strings to accept large integers

Timestamp's increment and timestamp components are unsigned 32-bit integers (max: 4294967295). On a 32-bit platform, 2147483647 will be the largest value accepted by the constructor since PHP's integers are signed.

It's possible for such values to exist on a 32-bit platform after unserializing a Timestamp from raw BSON, in which case it would be printed as a negative value (via ZVAL_LONG).

On a 32-bit platform, we should accept strings for these values, as we already do for UTCDateTime's milliseconds (a signed 64-bit integer). There are no Timestamp getter methods, so we need not worry about their return values. Strings can also be returned on 32-bit for var_dump(), serialization, and var_export() purposes (PHPC-460).