Support poking negative numbers.

Update dat_from_str() and doPokeBits() so they support negative
numbers.
FIR still broken, but Flo() values look better.

In order to get Flo litValues, Flo.fromNode() now uses
asTypeFor(). This requires more work.
FIR still broken, but Dbl() values look better.

Apply Flo changes to Dbl.
Reduce differences with master.
Simplify Lit.toLitVal().
Minor reformatting - tabs to spaces.
Add "tolerant" Float expect - accept single bit differences between expected and returned values.