add type signature to local binding (needed to build on GHC 7.7)
Adding type signature so it compiles with GHC7.7
bug fix: add special case for b==0 to quadform solver

Previously, it simply gave completely bogus results when b=0.

Fixes #88.
Merge pull request #89 from diagrams/issue88

bug fix: add special case for b==0 to quadform solver
Merge pull request #90 from co-dan/master

Adding type signature so it compiles with GHC7.7
Merge branch 'master' into parametric