Merge pull request #7 from PrikhodkoStanislav/hindley-milner

New tests for boolean and nil.
Merge pull request #8 from PrikhodkoStanislav/hindley-milner

Assignment.
Merge pull request #9 from PrikhodkoStanislav/hindley-milner

Identifier.
Merge pull request #10 from PrikhodkoStanislav/hindley-milner

Modified in visitor and tests.
added debug output
Merge remote-tracking branch 'origin/hindley-milner' into hindley-milner
fixed identifier
Merge pull request #5 from danilaml/hindley-milner

fixed identifier.