Lexer: Remove gratuitous differences of alex_scan_tkn' and alex_scan_tkn

Specifically, check_accs for both uses /exactly/ the same
implementation, but this is obscured by varying syntax:

  IBOX(e) <-> (I# e) and predx <-> pred
Lexer: update check_accs to match alex 3.0.3

This patch adapted from the alex project, commit fe081e8 (in the file
templates/GenericTemplate.hs).
Lexers: Remove Pred patterns for all context-less lexers.

check_accs needs to handle AlexAccPred and AlexAccSkipPred
Merge pull request #437 from jpaugh/master

Update to use alex >=v3.0.3