Makes memory limit configurable and a compile error (#419)

This allows users to reduce the memory limit per module below 4 Gi. This
is often needed because Wasm routinely leaves off the max, which implies
spec max (4 Gi). This uses Ki Gi etc in error messages because the spec
chooses to, though we can change to make it less awkward.

This also fixes an issue where we instantiated an engine inside config.

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

config_test.go

@@ -1,6 +1,7 @@
 package wazero
 
 import (
+	"context"
 	"io"
 	"math"
 	"testing"
@@ -11,7 +12,72 @@ import (
 	internalwasm "github.com/tetratelabs/wazero/internal/wasm"
 )
 
-func TestRuntimeConfig_Features(t *testing.T) {
+func TestRuntimeConfig(t *testing.T) {
+	tests := []struct {
+		name     string
+		with     func(*RuntimeConfig) *RuntimeConfig
+		expected *RuntimeConfig
+	}{
+		{
+			name: "WithContext",
+			with: func(c *RuntimeConfig) *RuntimeConfig {
+				return c.WithContext(context.TODO())
+			},
+			expected: &RuntimeConfig{
+				ctx: context.TODO(),
+			},
+		},
+		{
+			name: "WithContext - nil",
+			with: func(c *RuntimeConfig) *RuntimeConfig {
+				return c.WithContext(nil) //nolint
+			},
+			expected: &RuntimeConfig{
+				ctx: context.Background(),
+			},
+		},
+		{
+			name: "WithMemoryMaxPages",
+			with: func(c *RuntimeConfig) *RuntimeConfig {
+				return c.WithMemoryMaxPages(1)
+			},
+			expected: &RuntimeConfig{
+				memoryMaxPages: 1,
+			},
+		},
+		{
+			name: "mutable-global",
+			with: func(c *RuntimeConfig) *RuntimeConfig {
+				return c.WithFeatureMutableGlobal(true)
+			},
+			expected: &RuntimeConfig{
+				enabledFeatures: internalwasm.FeatureMutableGlobal,
+			},
+		},
+		{
+			name: "sign-extension-ops",
+			with: func(c *RuntimeConfig) *RuntimeConfig {
+				return c.WithFeatureSignExtensionOps(true)
+			},
+			expected: &RuntimeConfig{
+				enabledFeatures: internalwasm.FeatureSignExtensionOps,
+			},
+		},
+	}
+	for _, tt := range tests {
+		tc := tt
+
+		t.Run(tc.name, func(t *testing.T) {
+			input := &RuntimeConfig{}
+			rc := tc.with(input)
+			require.Equal(t, tc.expected, rc)
+			// The source wasn't modified
+			require.Equal(t, &RuntimeConfig{}, input)
+		})
+	}
+}
+
+func TestRuntimeConfig_FeatureToggle(t *testing.T) {
 	tests := []struct {
 		name          string
 		feature       internalwasm.Features