ISLE: support more flexible integer constants. (#4559)

The ISLE language's lexer previously used a very primitive
`i64::from_str_radix` call to parse integer constants, allowing values
in the range -2^63..2^63 only. Also, underscores to separate digits (as
is allwoed in Rust) were not supported. Finally, 128-bit constants were
not supported at all.

This PR addresses all issues above:
- Integer constants are internally stored as 128-bit values.
- Parsing supports either signed (-2^127..2^127) or unsigned (0..2^128)
  range. Negation works independently of that, so one can write
  `-0xffff..ffff` (128 bits wide, i.e., -(2^128-1)) to get a `1`.
- Underscores are supported to separate groups of digits, so one can
  write `0xffff_ffff`.
- A minor oversight was fixed: hex constants can start with `0X`
  (uppercase) as well as `0x`, for consistency with Rust and C.

This PR also adds a new kind of ISLE test that actually runs a driver
linked to compiled ISLE code; we previously didn't have any such tests,
but it is now quite useful to assert correct interpretation of constant
values.

cranelift/isle/isle/tests/run_tests.rs

@@ -18,7 +18,7 @@ pub fn run_fail(filename: &str) {
     assert!(build(filename).is_err());
 }
 
-pub fn run_link(isle_filename: &str) {
+fn build_and_link_isle(isle_filename: &str) -> (tempfile::TempDir, std::path::PathBuf) {
     let tempdir = tempfile::tempdir().unwrap();
     let code = build(isle_filename).unwrap();
 
@@ -45,10 +45,27 @@ pub fn run_link(isle_filename: &str) {
     let mut rustc = std::process::Command::new("rustc")
         .arg(&rust_driver)
         .arg("-o")
-        .arg(output)
+        .arg(output.clone())
         .spawn()
         .unwrap();
     assert!(rustc.wait().unwrap().success());
+
+    (tempdir, output)
+}
+
+pub fn run_link(isle_filename: &str) {
+    build_and_link_isle(isle_filename);
+}
+
+pub fn run_run(isle_filename: &str) {
+    let (_tempdir, exe) = build_and_link_isle(isle_filename);
+
+    assert!(std::process::Command::new(exe)
+        .spawn()
+        .unwrap()
+        .wait()
+        .unwrap()
+        .success());
 }
 
 // Generated by build.rs.