Auto-load additional .xtm files when using the Extempore backend

[oubiwann]

There are a handful of useful functions I've been using that aren't included in Extempore. They include:

• drop
• every-nth
• partition
• assoc-del
• assoc-upd

As well as some functions for converting numbers to arbitrary bases. Maybe these can go in src/util.xtm?

Furthermore, I've started adding additional chord names (some new, some aliases for existing chords. The data structure is named *ut:chord-patterns* and the function that uses them is ut:chord; these should be saved into their own pitch-class file ... maybe src/pitch.xtm?