Fix compilation for GHC-7.6.1
...and add top-level signature for `errmsg` to silence compiler warning
Fix lens generation when field names are imported qualified
Without this fix, if there's a field which is visible as A._f, the generated code looks like A.f = ... which leads to a compilation error.
Export a function that allows lenses to be generated directly from a Dec
Fix compilation with GHC 7.2.1
...in order to allow `template-haskell-184.108.40.206` which GHC 7.2.1 ships with
Renamed the TH functions and changed them to use '_' prefix.