fuzz: refactor fuzz generators (#4404)

Previously, much of the logic for generating the various objects needed
for fuzzing was concentrated primarily in `generators.rs`. In trying to
piece together what code does what, the size of the file and the light
documentation make it hard to discern what each part does. Since several
generator structures had been split out as separate modules in the
`generators/` directory, this change takes that refactoring further by
moving the structures in `generators.rs` to their own modules. No logic
changes were made, only the addition of documentation in a few places.

crates/fuzzing/src/generators/spec_test.rs

@@ -0,0 +1,29 @@
+//! Arbitrarily choose a spec test from the list of known spec tests.
+
+use arbitrary::{Arbitrary, Unstructured};
+
+// See `build.rs` for how the `FILES` array is generated.
+include!(concat!(env!("OUT_DIR"), "/spectests.rs"));
+
+/// A spec test from the upstream wast testsuite, arbitrarily chosen from the
+/// list of known spec tests.
+#[derive(Debug)]
+pub struct SpecTest {
+    /// The filename of the spec test
+    pub file: &'static str,
+    /// The `*.wast` contents of the spec test
+    pub contents: &'static str,
+}
+
+impl<'a> Arbitrary<'a> for SpecTest {
+    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+        // NB: this does get a uniform value in the provided range.
+        let i = u.int_in_range(0..=FILES.len() - 1)?;
+        let (file, contents) = FILES[i];
+        Ok(SpecTest { file, contents })
+    }
+
+    fn size_hint(_depth: usize) -> (usize, Option<usize>) {
+        (1, Some(std::mem::size_of::<usize>()))
+    }
+}