Replace mem:transmute in wrap_args with safer options

Replace `mem:transmute` in `wrap_args` with safer options as without it:

    $ frawk -Bcranelift 'BEGIN { printf("%d\t%f\t%s\n", integer, float, string); }'

would crash with:

    failure in runtime invalid type code passed to printf_impl_file: 2. Halting execution

or with `invalid opcode` after cranelift update.

Fixes: #121

Author: ghuls

src/codegen/intrinsics.rs

@@ -1144,10 +1144,16 @@ unsafe fn wrap_args<'a>(
                 ty_code
             )
         };
+
         let typed_arg: FormatArg = match ty {
-            Ty::Int => mem::transmute::<usize, Int>(arg).into(),
+            Ty::Int => (arg.cast_signed() as i64).into(),
             Ty::Float => Float::from_bits(arg as u64).into(),
-            Ty::Str => mem::transmute::<usize, &Str>(arg).clone().into(),
+            Ty::Str => {
+                unsafe {
+                    let str_ref = std::ptr::with_exposed_provenance::<Str>(arg).as_ref().unwrap();
+                    str_ref.clone().into()
+                }
+            },
             Ty::Null => FormatArg::Null,
             _ => fail!(
                 _rt,

src/compile.rs

@@ -25,6 +25,7 @@ use std::sync::Arc;
 pub(crate) const UNUSED: u32 = u32::max_value();
 pub(crate) const NULL_REG: u32 = UNUSED - 1;
 
+#[repr(u8)]
 #[derive(Copy, Clone, Eq, PartialEq, Debug, Hash, Default)]
 pub(crate) enum Ty {
     Int = 0,