token: Add None which represents a NONE option for tokens.
token: Reorganize code.
token: Pretty print the NONE option for tokens.
token: Use Kind to specify the NONE option for tokens; The zero value for tokens is still considered a NONE option.
ast: Initial commit of package ast, which declares the types used to represent abstract syntax trees of Go source code.
ast: Rename FunctionDecl to FuncDecl and add doc comments for Expr.