Haskell: skip multi-line type signature

[masatake]

Close #4013.

The original logic extract "thing" in

    thing
        :: App m
        => Int
        -> m Int

as a function because the parser cannot find : after "thing" in the line.

The test case is taken from #4013 submitted by @robinp.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.43%. Comparing base (08e07dc) to head (7456573).

Files	Patch %	Lines
parsers/haskell.c	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4019   +/-   ##
=======================================
  Coverage   85.43%   85.43%           
=======================================
  Files         235      235           
  Lines       56727    56734    +7     
=======================================
+ Hits        48462    48468    +6     
- Misses       8265     8266    +1     

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