expr: Simplify JimExprParseIrrationals

Signed-off-by: Steve Bennett <steveb@workware.net.au>
many comment changes, some small code changes

Sweep through and clean up all (most) of the comments in the code.
While there, adjust some variable and function names to be more consistent,
and make a few small code changes - again, mostly for consistency.

Signed-off-by: Steve Bennett <steveb@workware.net.au>