Search local polyml directory before PATH

If there is a local polyml directory, we probably want to use it.
Improve LinratAngleExpr tests

Also, some improvements to the LinratAngleExpr code; mostly formatting,
but also catching parser exceptions properly.
Reduce debug output in normal builds

It's mostly just distracting, and hides warnings.  local_config.ML can
be used to up the levels again for debugging particular problems.
Fix handling of invalid JSON LinratAngleExpr input

We had Null (a variable) instead of Json.Null (a value) as a match
expression, so invalid JSON types would get a zero expression instead of
an error.
Reduce warnings

Mostly just using _ for unused variables, but also making matches
exhaustive.