Simplify `parsers.rst.directives.get_measure()` by using
`nodes.parse_measure()`.
Sort the list of supported length units in the order used in the
rST specification (based on the CSS specification).
Update and extend the lenght value handling tests.
Clarify and test behaviour:
* case of length units is preserved (in contrast to CSS)
* the restriction of supported units is a rST feature
while the Docutils Doctree allows any run of ASCII letters + µ
or a percent sign as units on attributes with value type "measure".