Speed up loading .nkg files

[caiw]

Fixes #271

Loading over 5k functions from a .nkg file goes from 49 functions/second to *~928~ 1,331 functions/second**!

Thanks to @young-x-skyee for most of the work on this — I just cherry-picked commits from the kymata-language branch, and tidied up the code a little, and added/fixed tests

Main changes

• [x] New permitted and more performant argument format for ExpressionSet.__init__().
• [x] Update load_nkg to use new format to benefit from the speedups
• [x] Documentation for new format.
  • Added to ExpressionSet.__init__() docstring.
• [x] Tests for new argument format.
• [x] Refactored combine() to use new format and benefit from the speedups.
• [x] Refactored ExpressionSet.__add__() to use the new format and benefit from the speedups.

*: on my laptop

[neukym]

Loading over 5k functions from a .nkg file goes from 49 functions/second to 1,331 functions/second*!

Wow!