Add frozen_string_literal: true

[pocke]

RBS allocates many objects. It makes steep check slow.

So this PR reduces String allocations with frozen_string_literal: true magic comment.

Benchmarking

I benchmarked bin/steep -j1.

before: 
[13.82, 13.87, 13.73].sum / 3 # => 13.806666666666667

after:
[13.43, 13.27, 13.21].sum / 3 # => 13.303333333333333

(1 - 13.303333333333333 / 13.806666666666667) * 100 # => 3.645581844519563

3.6% faster 🚀

I've confirmed the effect with memory_profiler gem. It reduces 163918 strings (from 3711262 to 3547344).

[soutaro]

🧊 🙏