Coredump: add local/stack value typing

This change adds typing to local and stack values. Technically the type
isn't strictly necessary as DWARF should indicate how to interpret the values.

However, using types allows us to represent the lack of value, because
it has been optimized out and more complex values like reference types.

Closes WebAssembly#198

Coredump.md

@@ -95,16 +95,26 @@ For each thread a [Custom section], called `corestack`, is used to store the
 debugging information.
 
 ```
-corestack  ::= customsec(thread-info vec(frame))
+corestack   ::= customsec(thread-info vec(frame))
 thread-info ::= 0x0 thread-name:name
 frame       ::= codeoffset:u32
-                locals:vec(u32)
-                stack:vec(u32)
+                locals:vec(value)
+                stack:vec(value)
                 reserved:u32
 ```
 
 The `reserved` byte is decoded as an empty vector and reserved for future use.
 
+Local and stack values are encoded using [Value Types] (valtype) followed by bytes
+representing the actual value:
+```
+value ::= t:valtype x:t => x
+        | 0x01          => ∅
+```
+
+The special byte `0x01` is used to represent a missing value, usually because
+it was optimized out by the WebAssembly engine.
+
 ## Memory
 
 The process memory is captured in the [Data Section], either entirely as one
@@ -145,3 +155,4 @@ tooling: [demo].
 [Memory Section]: https://webassembly.github.io/spec/core/binary/modules.html#binary-memsec
 [instantiated]: https://webassembly.github.io/spec/core/exec/modules.html#instantiation
 [Global Section]: https://webassembly.github.io/spec/core/binary/modules.html#binary-globalsec 
+[Value Types]: https://webassembly.github.io/spec/core/binary/types.html#binary-valtype