ISLE: remove Scheme-style boolean syntax

The `#t` and `#f` syntax is misleading: they are actually integer
expressions, not booleans, so `#t` represents the integer `1` but *not*
the boolean `true`. They have been replaced by the named constants
`$true` and `$false`, and not used anywhere except tests, so they can be
removed without breaking any lowering/optimization code.

Copyright (c) 2024, Arm Limited.

Signed-off-by: Karl Meakin <karl.meakin@arm.com>

Author: Kmeakin

cranelift/isle/isle/isle_examples/pass/test4.isle

@@ -21,7 +21,7 @@
       a
       (Ext1 x)
       (Ext2 x)))
-  (C #t))
+  (C 1))
 
 (type Opcode (enum A B C))
 (type MachInst (enum D E F))

cranelift/isle/isle/src/parser.rs

@@ -890,10 +890,6 @@ impl<'a> Parser<'a> {
             let ret = self.parse_expr_inner_parens(pos)?;
             self.expect_rparen()?;
             Ok(ret)
-        } else if self.eat_sym_str("#t")? {
-            Ok(Expr::ConstInt { val: 1, pos })
-        } else if self.eat_sym_str("#f")? {
-            Ok(Expr::ConstInt { val: 0, pos })
         } else if self.is_const() {
             let val = self.parse_const()?;
             Ok(Expr::ConstPrim { val, pos })