parser debugging output: strip token name prefixes

[iabyn]

Some of the names of parser (perly.y) tokens have a common prefix, such as

PERLY_SEMICOLON
PERLY_AMPERSAND
KW_PACKAGE
KW_CLASS

Perl's -Dpv switch produces debugging output that also displays the top few items on the parse stack. The token names are truncated for compactness' sake. This currently leads to a display where its mostly just the token name's prefix that is displayed, e.g.

$ perl -Dpv -e'package Foo'

...

index: 1 2 3 4 5 6 7 8
state: 1 9 17 149 91 263 412 503
token: GRAMPROG @1 remember stmtseq KW_PACKA BAREWORD BAREWORD PERLY_SE
value: 0 0 63 (Nullop) 0 (Nullop) const 735909768

After this commit, PERLY_, KW_ etc prefixes are stripped, allowing more of the actual token name is displayed:

index: 1 2 3 4 5 6 7 8
state: 1 9 17 149 91 263 412 503
token: GRAMPROG @1 remember stmtseq PACKAGE BAREWORD BAREWORD SEMICOLO
value: 0 0 63 (Nullop) 0 (Nullop) const 227539304

• This set of changes does not require a perldelta entry.