- Separate library loading code into a new module. parser.__init__() no longer loads the library; it accepts (but does not depend on) a library as a parameter.
- Add an optional library argument to parse(). It's no longer mandatory to create a new parser for switching to a different builtins or seftable file.
- Move warning() and types from lslparse to lslcommon.
- Add .copy() to uses of base_keywords, to not rely on it being a frozen set.
- Adjust the test suite.
