Adds ability to disable mutable globals and improves decode perf (#315)

This adds `RuntimeConfig.WithFeatureMutableGlobal(enabled bool)`, which
allows disabling of mutable globals. When disabled, any attempt to add a
mutable global, either explicitly or implicitly via decoding wasm will
fail.

To support this, there's a new `Features` bitflag that can allow up to
63 feature toggles without passing structs.

While here, I fixed a significant performance problem in decoding
binary:

Before
```
BenchmarkCodecExample/binary.DecodeModule-16         	  184243	      5623 ns/op	    3848 B/op	     184 allocs/op
```

Now
```
BenchmarkCodecExample/binary.DecodeModule-16         	  294084	      3520 ns/op	    2176 B/op	      91 allocs/op

```

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

internal/wasm/global_test.go

@@ -1,7 +1,6 @@
 package internalwasm
 
 import (
-	"context"
 	gobinary "encoding/binary"
 	"testing"
 
@@ -257,7 +256,7 @@ func TestPublicModule_Global(t *testing.T) {
 	for _, tt := range tests {
 		tc := tt
 
-		s := NewStore(context.Background(), &catchContext{})
+		s := newStore()
 		t.Run(tc.name, func(t *testing.T) {
 			// Instantiate the module and get the export of the above global
 			module, err := s.Instantiate(tc.module, "")