Merge pull request #2 from player1537/master

Fixed syntax problem for issue #1
starts_with now accepts regex, other fixes
reflecting recent changes
Renamed find() to index_of()

"index_of()" better reflects the nature of the function and further distinguishes it from search()
performance improvements to regex functions

invocations of _compile_regex() now moved to public functions
Renamed find() to index_of()
Update strings.scad
Renamed find() to index_of()
Moving towards working split() for regex
split() works for regex, now
Added alternate regex engine using trees

this should resolve efficiency issues associated with performing _match_prefix() for every binary regex operation in a regex.
new tests for _match_regex_tree()
switched public regex methods to _match_regex_tree
added tests for _ignore_case on _match_regex_tree
more tests for _prefix_to_tree()
moved _infix_to_prefix to use list based stack

a list based stack affords storing multi character strings, as well as other more complex data structures
ditto, postfix
explicit io stack in _infix_to_postfix()

This change is made in prep for a function that converts directly from infix notation to a parse tree, which will require the added versatility.
reimplemented _prefix_to_tree for efficiency

New method processes the prefix in a manner similar to machine code, with the input register represented through the call stack. This implementation no longer requires a node in the parse tree to be read twice.
dropped _infix_to_prefix for _regex_to_tree

this change allows straight conversion from primed regex to tree structures.