Use PointerMutability inside Type::Ptr

[dtolnay]

Currently, Expr::RawAddr is represented using PointerMutability enum:

syn/src/expr.rs

Lines 583 to 589 in da478ac

	pub struct ExprRawAddr #full {
	pub attrs: Vec<Attribute>,
	pub and_token: Token![&],
	pub raw: Token![raw],
	pub mutability: PointerMutability,
	pub expr: Box<Expr>,
	}

whereas Type::Ptr is represented using separate Option<Token![const]> and Option<Token![mut]> fields.

syn/src/ty.rs

Lines 183 to 188 in da478ac

	pub struct TypePtr {
	pub star_token: Token![*],
	pub const_token: Option<Token![const]>,
	pub mutability: Option<Token![mut]>,
	pub elem: Box<Type>,
	}