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/isle_examples/run/iconst.isle

@@ -0,0 +1,17 @@
+(type i64 (primitive i64))
+
+(decl X (i64) i64)
+(rule (X -1) -2)
+(rule (X -2) -3)
+(rule (X 0x7fff_ffff_ffff_ffff) 0x8000_0000_0000_0000)
+(rule (X 0xffff_ffff_ffff_fff0) 1)
+
+(type i128 (primitive i128))
+
+(decl Y (i128) i128)
+
+(rule (Y 0x1000_0000_0000_0000_1234_5678_9abc_def0) -1)
+(rule (Y 0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff) 3)
+
+(rule (Y -0x1000_0000_0000_0000_1234_5678_9abc_def0) 1)
+(rule (Y -0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff) -3)

cranelift/isle/isle/isle_examples/run/iconst_main.rs

@@ -0,0 +1,18 @@
+mod iconst;
+
+struct Context;
+impl iconst::Context for Context {}
+
+fn main() {
+    let mut ctx = Context;
+
+    assert_eq!(iconst::constructor_X(&mut ctx, -1), Some(-2));
+    assert_eq!(iconst::constructor_X(&mut ctx, -2), Some(-3));
+    assert_eq!(iconst::constructor_X(&mut ctx, 0x7fff_ffff_ffff_ffff), Some(0x8000_0000_0000_0000u64 as i64));
+    assert_eq!(iconst::constructor_X(&mut ctx, 0xffff_ffff_ffff_fff0_u64 as i64), Some(1));
+
+    assert_eq!(iconst::constructor_Y(&mut ctx, 0x1000_0000_0000_0000_1234_5678_9abc_def0), Some(-1));
+    assert_eq!(iconst::constructor_Y(&mut ctx, 0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffffu128 as i128), Some(3));
+    assert_eq!(iconst::constructor_Y(&mut ctx, -0x1000_0000_0000_0000_1234_5678_9abc_def0), Some(1));
+    assert_eq!(iconst::constructor_Y(&mut ctx, -(0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffffu128 as i128)), Some(-3));
+}