HsLexer does not recognize _var as valid

HsLexer does not recognize _var as a valid pattern, for
instance the following leads to a parse error:

case apa of
  _bepa -> cepa

I've attached a patch that fixes this, but it's not
very nice. At least it shows the problem.

The Haskell report allows _ at the start of vars and
considers it lower case.

See 2.2 of the Haskell report, _ is not treated
differently than other small characters. The only
special case about _ is that _ is a reservedid, so it
should really be parsed as such (KW_Underscore for
instance). This requires changes in the parser, my
patch only changes the lexer (not changing the data type).

Trac metadata

Trac field	Value
Version	5.0
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	ResolvedFixed
Component	hslibs/hssource
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