remove unused includes, simplify alloc/free

Fix loss of precision when converting string to value.

Revert to static size for our buffers.

64 should be more than enough, never could get strings longer than ~21 chars so far.

Add more tests to bpyunits.

Only LENGTH type for now.

This showed two limits to current BKE's units code:
* When converting string to value, we use too few precision for units' values (6 digits),
not good esp. when mixing big and small units (like e.g. miles and inches). Will fix that
in next commit.
* When converting values to string, 'precision' parameter ignores order of magnitude of the value,
which leads to invalid results with big numbers (e.g. 1000000mi gives 1000000.004mi with prec of 3).

Fix doc strings.

Deduplicate some code for struct-seq initialization

Make units a real submodule.

Minor cleanup/changes to api & docstrings

Merge branch 'master' into temp-pyapi-units

Some minor tweaking on temps buffers...

Modifications from Campbell's review, thanks! :)

First step toward a basic units py API (exposes all supported unti systems & types, and str_to_value/value_to_str functions).

Also adds a skeletton of unittest file for it!

Reviewers: campbellbarton

CC: carter2422

Differential Revision: https://developer.blender.org/D416