Split up parser

The structure mostly follows the ast structure, with some slight
changes. Each parser submodule documents which ast submodule it
corresponds to.

This parser is not yet complete, and I have yet to go through its
modules one-by-one to fix and complete them.

src/parser/basic.rs

@@ -0,0 +1,23 @@
+//! Corresponds to `ast::basic`.
+
+use chumsky::prelude::*;
+
+use crate::ast::{Ident, Space};
+use crate::span::Span;
+
+pub type Error = Simple<char, Span>;
+
+// TODO https://github.com/rust-lang/rust/issues/63063
+
+pub fn space() -> impl Parser<char, Space, Error = Error> {
+    // TODO Parse comments
+    text::whitespace().map_with_span(|(), span| Space {
+        comment: vec![],
+        span,
+    })
+}
+
+pub fn ident() -> impl Parser<char, Ident, Error = Error> {
+    // TODO Forbid keywords
+    text::ident().map_with_span(|name, span| Ident { name, span })
+}