semantic analysis: document validation

[lrlna]

Context

Stemming from #144, this issue describes document semantic analysis and validation. As an outcome of Document validation, we must ensure that a document is semantically correct, and provide users with diagnostics if it's not. Additionally we must have a user-facing source database with straightforward access to various parts of a document, such as its defined arguments, or fragments.

The following pieces of grammar are to be validated:

• semantic analysis: validate operation definition in Document #149
• semantic analysis: validate operation Fragment and its Type Condition in Document #150
• semantic analysis: validate Input Values in Document #151
  • String Value
  • List Value
  • Input Object Value
  • Varibale
  • Named Type
• Arguments
• Directives

[lrlna]

done!