Since the Z values are normalized (which ordinarily mutates them as
needed) before checking for equality, the race detector gets confused when
using a global value for the field representation of the value 1 and
passing it into the various internal arithmetic routines and reports a
false positive.
Even though the race was a false positive and had no adverse effects, this
commit silences the race detector by creating new variables at the top
level and passing them instead of the global fieldOne variable. The
global is still used for comparison operations since those have no
potential to mutate the value and hence don't trigger the race detector.
This change adds an additional signature parsing function which
performs additional checks to verify the signature is serialized in a
valid DER (and thus, unique) format, instead of allowing the less
strict BER signatures that ParseSignature will happily accept.
Added additional tests and updated test coverage to reflect changes.
