Flatten nested Values with associative transformers

New Features

• Add many Value creation functions, like float_values.min_float, int_values.sum_ints and str_values.concatenate
• Add StrValue.length -> IntValue
• Allow caching of StrConstants with StrConstant.of("foo", cache=True)

Improvements

• Adding, Multiplying, concatenating and many other associative operations on nested Values will now be flattened. This will reduce complexity of derived Value structures
• Values derived entirely of constants or literals are now themselves constants. So IntConstant(3) + IntConstant(5) == IntConstant(8). This will reduce complexity of derived Value structures in some situations
• Caching some constants, like ints in range [-100, 100], floats in range [-100.0, 100.0], bools {True, False} and some select single str chars
• Other small performance improvements

Bug fixes

None

Breaking Changes

• Removed many classes, like ClampFloatValue, which have been replaced more general classes, like ThreeFloatToFloatValue