[wiggle] Impl different formatters for flags (#1299)

* Impl different formatters for flags

Rather than forcing only binary formatting of flags types, how about
we implement all relevant traits (`Binary`, `Octal`, `LowerHex`, and
`UpperHex`) and allow the user to pick the most relevant one for their
use case?

Also, we use at least `Octal` and `LowerHex` in a couple of places
in `wasi-common`.

* fmt::Display for flags now inspired by bitflags

Flags is now by default formatted similarly to how
`bitflags` crate does it, namely, `dsync|append (0x11)`. In case
we're dealing with an empty set, we get `empty (0x0)`. Because of
this, any `Octal`, `LowerHex`, etc., formatters are redundant now.

Furthermore, while here, I've rewritten `EMPTY_FLAGS` and `ALL_FLAGS`
(where the former means `0x0` and the latter is the union of all possible
values) to be `const fn empty()` and `const fn all()` where the latter is
an expanded union of primitive representation values out of a macro.
This is again largely inspired by the `bitflags` crate.

* Test fmt::Display for flags

crates/wiggle/tests/flags.rs

@@ -25,7 +25,7 @@ impl<'a> flags::Flags for WasiCtx<'a> {
 }
 
 fn car_config_strat() -> impl Strategy<Value = types::CarConfig> {
-    (1u8..=types::CarConfig::ALL_FLAGS.into())
+    (1u8..=types::CarConfig::all().into())
         .prop_map(|v| {
             types::CarConfig::try_from(v).expect("invalid value for types::CarConfig flag")
         })
@@ -99,3 +99,15 @@ proptest! {
         e.test()
     }
 }
+
+#[test]
+fn flags_fmt() {
+    let empty = format!("{}", types::CarConfig::empty());
+    assert_eq!(empty, "empty (0x0)");
+    let one_flag = format!("{}", types::CarConfig::AWD);
+    assert_eq!(one_flag, "awd (0x2)");
+    let two_flags = format!("{}", types::CarConfig::AUTOMATIC | types::CarConfig::SUV);
+    assert_eq!(two_flags, "automatic|suv (0x5)");
+    let all = format!("{}", types::CarConfig::all());
+    assert_eq!(all, "automatic|awd|suv (0x7)");
+}