Internal code reorganization.

- 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.

lslopt/lslfoldconst.py

@@ -18,11 +18,10 @@
 # Constant folding and simplification of expressions and statements.
 
 import lslcommon
-from lslcommon import Vector, Quaternion
+from lslcommon import Vector, Quaternion, warning
 import lslfuncs
 from lslfuncs import ZERO_VECTOR, ZERO_ROTATION
 import math
-from lslparse import warning
 from lslfuncopt import OptimizeFunc, OptimizeArgs, FuncOptSetup