API|Uri: Added new Uri constructor Uri_NewWithPath3

A default scheme is set for the new Uri before then parsing a cstring
representation of a Uri, bypassing the resource class mechanism.

This provides a convenient method of constructing a Uri with a default
scheme that may be overridden with the cstring argument.
ACS|Cleanup: Use Uri_NewWithPath3
libcommon: Provide a Reader/Writer to all save state (de)serialization routines
Refactor|ACS: Implement ACScript (de)serialization in C++ with Reader/Writer

Also, ACScript_Thinker() now calls the C++ method ACScript::runTick()