Bug 1375930

Previously, the `str_replace` intrinsic function worked by calling the Python `str.replace()` method for each string to be replaced. Consequently, if the replacement text for one replacement contained another of the strings to be replaced, the replacement text itself could be replaced. The result was non-deterministic, since the replacement order was not guaranteed. Therefore users had to be careful to use techniques, such as guard characters, to ensure that there was no misinterpretation. With this update, replacements are now performed in a single pass, so only the original text is subject to replacement. As a result, the output of `str_replace` is now deterministic, and consistent with user expectations even without the use of guard characters. When keys overlap in the input, longer matches are preferred. Lexicographically smaller strings will be replaced first if there is still ambiguity.