Tracking issue for `runtime_pattern!` macro

[Lancern]

As discussed in #45, a RuntimePattern implementation with a pattern registry that can be dynamically built may have security concerns. Thus, while the pattern template string can be evaluated dynamically, the pattern registry, which is basically a map from string to pattern factories, needs to be built at compile time.

We thus propose a new macro runtime_pattern! for building runtime patterns. It can be used as follows:

let pattern: RuntimePattern = runtime_pattern!(template_str, {$custom} => Custom::default);

The only difference between pattern! and runtime_pattern! is that while pattern! only accept a string literal, runtime_pattern! accepts any expressions that evaluate to a string.

This issue tracks the development progress of runtime_pattern!.