[[ CopyOnWriteStrings ]] Initial implementation
Destroy string appropriately
[[ CopyOnWriteStrings ]] Remove unneccessary clone when copying mutable -> immutable
Make sure MCStringCopyAndRelease returns a value
Fix some memory leaks
Merge branch 'refactor-syntax_unicode' of https://www.github.com/runrev/livecode into refactor-syntax_unicode_copy_on_write

Conflicts:
	libfoundation/src/foundation-string.cpp
Merge pull request #632 from runrevali/refactor-syntax_unicode_copy_on_write

Refactor syntax unicode copy on write