Implement char type in adapter fusion (#4544)

This commit implements the translation of `char` which validates that
it's in the valid range of unicode scalar values. The precise validation
here is lifted from LLVM in the hopes that it's probably better than
whatever I would concoct by hand.

crates/environ/fuzz/fuzz_targets/fact-valid-module.rs

@@ -51,6 +51,7 @@ enum ValType {
     S64,
     Float32,
     Float64,
+    Char,
     Record(Vec<ValType>),
     Tuple(Vec<ValType>),
     Variant(NonZeroLenVec<ValType>),
@@ -206,6 +207,7 @@ fn intern(types: &mut ComponentTypesBuilder, ty: &ValType) -> InterfaceType {
         ValType::S64 => InterfaceType::S64,
         ValType::Float32 => InterfaceType::Float32,
         ValType::Float64 => InterfaceType::Float64,
+        ValType::Char => InterfaceType::Char,
         ValType::Record(tys) => {
             let ty = TypeRecord {
                 fields: tys