refactor(mir): phase260 p0 edge-args plumbing (strangler) + ssot api + docs

tests/wasm_string_constants.rs

@@ -146,7 +146,7 @@ fn build_string_const_mir_module() -> MirModule {
     });
 
     // return %str
-    block.terminator = Some(MirInstruction::Return {
+    block.set_terminator(MirInstruction::Return {
         value: Some(str_value),
     });
 
@@ -193,7 +193,7 @@ fn build_multiple_string_const_mir_module() -> MirModule {
     });
 
     // return %str1
-    block.terminator = Some(MirInstruction::Return {
+    block.set_terminator(MirInstruction::Return {
         value: Some(str1_value),
     });