PHP-1245: Use php_mongocode_populate instead of a MONGO_METHOD.
Added missing return type.

Otherwise it defaults to int... and that's not what we want.
Merged pull request #790
PHP-1245: Put MongoCursor::next in its own function, so we can use it in explain().

And add a basic test case for explain().
Merged pull request #791
PHP-953: Do not check custom or generated index name lengths

The 127-byte limit on index names is specific to mmapv1 storage. For both commands and legacy inserts (to system.indexes), we can rely on the server to validate the length accordingly.

Added tests for legacy inserts to ensure an exception is still thrown (albeit with a different message directly from the server).