Fix for #659 variable lookup confusion

[opeongo]

• Change the way the way that the block namespace cache is implemented. Previously it used an imprecise equals method that led to collisions and occasionally returned null values.

• Move the namespace swap out of the iterative loop in BSHEnhancedForLoop to avoid unnecessary lookups.

[codecov[bot]]

Codecov Report

Attention: 661 lines in your changes are missing coverage. Please review.

Comparison is base (b1998b1) 74.24% compared to head (5a3fb21) 74.19%. Report is 1024 commits behind head on master.

Files	Patch %	Lines
src/main/java/bsh/Interpreter.java	85.20%	50 Missing and 21 partials :warning:
src/main/java/bsh/BshClassManager.java	65.53%	48 Missing and 13 partials :warning:
src/main/java/bsh/NameSpace.java	87.87%	40 Missing and 17 partials :warning:
src/main/java/bsh/ClassGeneratorUtil.java	89.59%	35 Missing and 14 partials :warning:
src/main/java/bsh/BshMethod.java	76.73%	26 Missing and 21 partials :warning:
src/main/java/bsh/Name.java	86.76%	20 Missing and 23 partials :warning:
src/main/java/bsh/LHS.java	74.35%	28 Missing and 12 partials :warning:
src/main/java/bsh/Invocable.java	83.03%	23 Missing and 15 partials :warning:
src/main/java/bsh/DelayedEvalBshMethod.java	61.97%	26 Missing and 1 partial :warning:
src/main/java/bsh/BSHPrimarySuffix.java	84.41%	17 Missing and 7 partials :warning:
... and 30 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #741 +/- ## ============================================ - Coverage 74.24% 74.19% -0.05% + Complexity 3041 3038 -3 ============================================ Files 108 108 Lines 9354 9313 -41 Branches 1857 1862 +5 ============================================ - Hits 6945 6910 -35 + Misses 2070 2062 -8 - Partials 339 341 +2 ``` | [Flag](https://app.codecov.io/gh/beanshell/beanshell/pull/741/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beanshell) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/beanshell/beanshell/pull/741/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beanshell) | `74.19% <87.35%> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beanshell#carryforward-flags-in-the-pull-request-comment) to find out more.

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[opeongo]

Can we have some discussion on this or perhaps get it merged?