Port for purescript 0.11

[zudov]

Updates needed to make it compile, work and pass the tests with 0.11 branch of compiler.

Note: this doesn't involve the changes needed to support corefn AST outputted by 0.11, that would be the next step.

purescript-foreign have now removed all the json-related functionality for good, so the biggest change is the switch to purescript-argonaut.

• instead of concrete json-decoding functions (e.g. readString, readInt, readIdent, readExpr) polymorphic decodeJson is used in many places now.
  • to retain the explicitness types are specified when the result of decodeJson is binded in the do-notation
• errors are now communicated through Either String
  • typed error representation might be a subject of another commit
  • lmap is used to annotate error messages that come from parsers allowing to 'trace' a path at which the failure occured.

[paulyoung]

Hi @zudov. Thanks for taking a stab at this.

I'm going to close this PR since I was able to achieve 0.11 support in #40 with fewer changes. It also doesn't introduce the argonaut dependency or change the API.