py/lexer: Simplify and reduce code size for operator tokenising.

By removing the 'E' code from the operator token encoding mini-language the
tokenising can be simplified.  The 'E' code was only used for the !=
operator which is now handled as a special case; the optimisations for the
general case more than make up for the addition of this single, special
case.  Furthermore, the . and ... operators can be handled in the same way
as != which reduces the code size a little further.

This simplification also removes a "goto".

Changes in code size for this patch are (measured in bytes):

bare-arm:       -48
minimal x86:    -64
unix x86-64:   -112
unix nanbox:    -64
stmhal:         -48
cc3200:         -48
esp8266:        -76