Support local variables access of external scopes in Kernel#eval.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7aa5cfe) 90.86% compared to head (3240556) 90.89%. Report is 2 commits behind head on master.

Files	Patch %	Lines
monoruby/src/bin/irm/main.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #34 +/- ## ========================================== + Coverage 90.86% 90.89% +0.03% ========================================== Files 98 98 Lines 31194 31305 +111 ========================================== + Hits 28343 28454 +111 Misses 2851 2851 ```

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