Make ast::Name a type wrapper that checks validity

[SimonSapin]

This is breaking change that we should either make before 1.0 leaves beta, or decide not to make.

---

In 1.0.0-beta.4 apollo_compiler::ast::Name is a type alias for apollo_compiler::NodeStr, a smart string type with infallible conversion from arbitrary &str. As a result it is possible to generate invalid syntax:

let mut doc = apollo_compiler::ast::Document::parse("{ field }", "input.graphql");
if let apollo_compiler::ast::Definition::OperationDefinition(op) = &mut doc.definitions[0] {
    op.make_mut().name = Some("++".into())
}
assert_eq!(
    doc.serialize().no_indent().to_string(),
    "query ++ { field }"
)

This simple example uses AST, but even full validation in Schema or ExecutableDocument would not catch this.

We could change validation to find every Name and check its validity (must be made of one NameStart character followed by zero or more NameContinue characters) but it would be easy for validation code to miss some, and delay finding a problem until the user actually calls validation.

Proposal

Instead, ast::Name could be changed to a wrapper type of NodeStr that checks validity. Conversion from &str would be fallible. To avoid run-time unwrap when converting from a literal string, there could be a name!("example") macro that checks validity at compile-time.