Moved from the crate root to the new(ly public) `apollo_compiler::parser` module:
* `Parser`
* `SourceFile`
* `SourceMap`
* `FileId`

Moved to the `parser` module and renamed:
* `NodeLocation` → `SourceSpan`
* `GraphQLLocation` → `LineColumn`

Added a few helper methods:
* `executable::OperationMap::from_one`
* `schema::SchemaDefinition::iter_root_operations`
* `schema::ExtendedType::is_leaf`