rollback stack before executing ensure clause.
catching NameError here is not desirable.
Minor code format changes.
Fix typo.
Merge pull request #1184 from iij/pr-ensure-on-correct-stack

Rollback stack before executing ensure clause.
Merge pull request #1186 from carsonmcdonald/mrbcfixup

Minor format changes and a typo fix
Add mruby-proc-ext
Merge pull request #1187 from h2so5/mruby-proc-ext

Add mruby-proc-ext