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>

config_test.go

@@ -0,0 +1,48 @@
+package wazero
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	internalwasm "github.com/tetratelabs/wazero/internal/wasm"
+)
+
+func TestRuntimeConfig_Features(t *testing.T) {
+	tests := []struct {
+		name          string
+		feature       internalwasm.Features
+		expectDefault bool
+		setFeature    func(*RuntimeConfig, bool) *RuntimeConfig
+	}{
+		{
+			name:          "mutable-global",
+			feature:       internalwasm.FeatureMutableGlobal,
+			expectDefault: true,
+			setFeature: func(c *RuntimeConfig, v bool) *RuntimeConfig {
+				return c.WithFeatureMutableGlobal(v)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		tc := tt
+
+		t.Run(tc.name, func(t *testing.T) {
+			c := NewRuntimeConfig()
+			require.Equal(t, tc.expectDefault, c.enabledFeatures.Get(tc.feature))
+
+			// Set to false even if it was initially false.
+			c = tc.setFeature(c, false)
+			require.False(t, c.enabledFeatures.Get(tc.feature))
+
+			// Set true makes it true
+			c = tc.setFeature(c, true)
+			require.True(t, c.enabledFeatures.Get(tc.feature))
+
+			// Set false makes it false again
+			c = tc.setFeature(c, false)
+			require.False(t, c.enabledFeatures.Get(tc.feature))
+		})
+	}
+}