Fixes global numeric types to have max of signed encoding (#442)

This adjusts towards the exiting code which used int32/64 instead of
uint32/64. The reason is that the spec indicates intepretation as signed
numbers, which affects the maximum value.

See https://www.w3.org/TR/wasm-core-1/#value-types%E2%91%A2

Signed-off-by: Adrian Cole <adrian@tetrate.io>

api/wasm.go

@@ -31,9 +31,13 @@ import (
 type ValueType = byte
 
 const (
+	// ValueTypeI32 is a 32-bit integer.
 	ValueTypeI32 ValueType = 0x7f
+	// ValueTypeI64 is a 64-bit integer.
 	ValueTypeI64 ValueType = 0x7e
+	// ValueTypeF32 is a 32-bit floating point number.
 	ValueTypeF32 ValueType = 0x7d
+	// ValueTypeF64 is a 32-bit floating point number.
 	ValueTypeF64 ValueType = 0x7c
 )