Introduce custom type StringWithAltValues

[chrismarget-j]

StringWithAltValues is a custom type which supports semantic equality.

foo := NewStringWithAltValuesValue("foo", "bar", "bar") // alternate values: "bar" and "bar"
bar := NewStringWithAltValuesValue("bar")
foo.Equal(bar) // false
foo.StringSemanticEquals(ctx, bar) // true

My hope is to use this type in cases like leaf=>VN bindings where the API accepts a leaf switch ID but returns the leaf redundancy group ID.

Closes #990