Assorted API tweaks + prepare apollo_compiler@1.0.0 beta.19

[SimonSapin]

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

[SimonSapin]

New crate root:

parser module: