Adds Runtime.WithCapacityPages to avoid allocations during runtime. (#514)

`Runtime.WithMemoryCapacityPages` is a function that determines memory
capacity in pages (65536 bytes per page). The inputs are the min and
possibly nil max defined by the module, and the default is to return
the min.

Ex. To set capacity to max when exists:
```golang
c.WithMemoryCapacityPages(func(minPages uint32, maxPages *uint32) uint32 {
	if maxPages != nil {
		return *maxPages
	}
	return minPages
})
```

Note: This applies at compile time, ModuleBuilder.Build or Runtime.CompileModule.

Fixes #500

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

config_test.go

@@ -18,12 +18,12 @@ func TestRuntimeConfig(t *testing.T) {
 		expected *RuntimeConfig
 	}{
 		{
-			name: "WithMemoryMaxPages",
+			name: "WithMemoryLimitPages",
 			with: func(c *RuntimeConfig) *RuntimeConfig {
-				return c.WithMemoryMaxPages(1)
+				return c.WithMemoryLimitPages(1)
 			},
 			expected: &RuntimeConfig{
-				memoryMaxPages: 1,
+				memoryLimitPages: 1,
 			},
 		},
 		{
@@ -83,6 +83,24 @@ func TestRuntimeConfig(t *testing.T) {
 			require.Equal(t, &RuntimeConfig{}, input)
 		})
 	}
+
+	t.Run("WithMemoryCapacityPages", func(t *testing.T) {
+		c := NewRuntimeConfig()
+
+		// Test default returns min
+		require.Equal(t, uint32(1), c.memoryCapacityPages(1, nil))
+
+		// Nil ignored
+		c = c.WithMemoryCapacityPages(nil)
+		require.Equal(t, uint32(1), c.memoryCapacityPages(1, nil))
+
+		// Assign a valid function
+		c = c.WithMemoryCapacityPages(func(minPages uint32, maxPages *uint32) uint32 {
+			return 2
+		})
+		// Returns updated value
+		require.Equal(t, uint32(2), c.memoryCapacityPages(1, nil))
+	})
 }
 
 func TestRuntimeConfig_FeatureToggle(t *testing.T) {