Add fuzzing to the parser.

As part of this, todos and panics where moved outside of this module
to elsewhere when they would cause fuzzing to fail.

compiler/parse/src/test_helpers.rs

@@ -0,0 +1,22 @@
+use crate::ast::{self, Attempting};
+use crate::blankspace::space0_before;
+use crate::expr::expr;
+use crate::parser::{loc, Fail, Parser, State};
+use bumpalo::Bump;
+use roc_region::all::Located;
+
+#[allow(dead_code)]
+pub fn parse_with<'a>(arena: &'a Bump, input: &'a str) -> Result<ast::Expr<'a>, Fail> {
+    parse_loc_with(arena, input).map(|loc_expr| loc_expr.value)
+}
+
+#[allow(dead_code)]
+pub fn parse_loc_with<'a>(arena: &'a Bump, input: &'a str) -> Result<Located<ast::Expr<'a>>, Fail> {
+    let state = State::new(input.trim().as_bytes(), Attempting::Module);
+    let parser = space0_before(loc(expr(0)), 0);
+    let answer = parser.parse(&arena, state);
+
+    answer
+        .map(|(loc_expr, _)| loc_expr)
+        .map_err(|(fail, _)| fail)
+}