Add pending spec for Docile#dsl_eval_immutable
First light on Docile#dsl_eval_immutable
Refactor to share common code between #dsl_eval and #dsl_eval_immutable
Add specs of frozen string and number for #dsl_eval_immutable
Add documentation of #dsl_eval_immutable
Refactor #exec_in_proxy_context into private module Execution
Mark proxy classes as @api private
Improve documentation
Add documentation to README about #dsl_eval_immutable