Exports wasm.ValueType and makes Functions interfaces for signature verification (#284)

This moves code for `ValueType` constants public, so that we can
centralize discussion on how values are encoded. We'll need this when
re-introducing a Globals API.

To keep complexity down in consideration that there are only 4 types, I
copied the constants into the internalwasm package. This reduces the
import complexity otherwise would have caused.

Signed-off-by: Adrian Cole <adrian@tetrate.io>
Co-authored-by: Takaya Saeki <takaya@tetrate.io>

wasm_test.go

@@ -214,7 +214,7 @@ func TestFunction_Context(t *testing.T) {
 			require.NoError(t, err)
 
 			// This fails if the function wasn't invoked, or had an unexpected context.
-			results, err := exports.Function(functionName)(tc.ctx)
+			results, err := exports.Function(functionName).Call(tc.ctx)
 			require.NoError(t, err)
 			require.Equal(t, expectedResult, results[0])
 		})