Missing space
Set 'standard math mode' defaults for plane angle symbols

At the level of this module, it makes most sense to keep the
definitions quite basic: potentially a slightly different set will apply
in the higher level set up.
Standard math mode default for degree Celsius

A bit of trickery to get everything correct!
Normally module prefixes are singular
Reverse order of part/'set' in public functions

The general pattern for expl3 is \<thing>_set:Nn, and here <thing>
makes most sense as 'unit'/'unit_prefix'/'unit_power'/'unit_qualifier'.
Also removes the redundant second 'unit' for the unit symbols
themselves.
Use standard symbol for angstrom

This one is a bit tricky as it's got to be in text mode, at least
as standard. Provided the 'higher level' code to deal with the font
is correct this will work fine: will need testing!