Intepreter and test updates from @rossberg (#99)

Interpreter:

- Fixed evaluation of v128 load/store instructions to work with i64
- Reworked bulk operation execution to still reduce to well-typed instructions for i32
- Added missing size check to table allocation
- Various minor refactorings and clean-ups

Tests:

- Added tests for size check in i64 table and memory type limits

Split out from WebAssembly/spec#1839

interpreter/binary/decode.ml

@@ -281,19 +281,19 @@ let limits uN s =
   let flags = byte s in
   require (flags land 0xfa = 0) s (pos s - 1) "malformed limits flags";
   let has_max = (flags land 1 = 1) in
-  let is64 = (flags land 4 = 4) in
+  let at = if flags land 4 = 4 then I64AT else I32AT in
   let min = uN s in
   let max = opt uN has_max s in
-  {min; max}, is64
+  at, {min; max}
 
 let table_type s =
   let t = ref_type s in
-  let lim, is64 = limits u64 s in
-  TableT (lim, (if is64 then I64AddrType else I32AddrType), t)
+  let at, lim = limits u64 s in
+  TableT (at, lim, t)
 
 let memory_type s =
-  let lim, is64 = limits u64 s in
-  MemoryT (lim, if is64 then I64AddrType else I32AddrType)
+  let at, lim = limits u64 s in
+  MemoryT (at, lim)
 
 let global_type s =
   let t = val_type s in

interpreter/binary/encode.ml

@@ -194,15 +194,15 @@ struct
     | RecT [st] -> sub_type st
     | RecT sts -> s7 (-0x32); vec sub_type sts
 
-  let limits vu {min; max} at =
-    let flags = flag (max <> None) 0 + flag (at = I64AddrType) 2 in
-    byte flags; vu min; opt vu max
+  let limits at {min; max} =
+    let flags = flag (max <> None) 0 + flag (at = I64AT) 2 in
+    byte flags; u64 min; opt u64 max
 
   let table_type = function
-    | TableT (lim, at, t) -> ref_type t; limits u64 lim at
+    | TableT (at, lim, t) -> ref_type t; limits at lim
 
   let memory_type = function
-    | MemoryT (lim, at) -> limits u64 lim at
+    | MemoryT (at, lim) -> limits at lim
 
   let global_type = function
     | GlobalT (mut, t) -> val_type t; mutability mut