This removes some copypasta from existing Database and Collection code.

Also change "read-only" language to "immutable".

This obviates the need to handle stdClass instances within the results.

PHPC-224 consolidated the Result and Cursor classes into one. Additionally, PHPC-203 changed toArray() to return iterator_to_array($this) instead of only the first result document. For commands, this means we need to extract the first element in toArray()'s return value.

This also changes the existing collection functional test to inherit FunctionTestCase

Inheriting from the Client's URI options is not implemented, as it depends on PHPC-196.

This commit also revises the documentation for selectDatabase(), selectCollection(), and the core class constructors.