Move LoopScopes into ExprContext

[seanyoung]

This simplifies the code, and makes it to use ExprContext for other purposes (for example variable scoping).

[codecov[bot]]

Codecov Report

Attention: 49 lines in your changes are missing coverage. Please review.

Comparison is base (8c4111b) 87.74% compared to head (8190ca8) 87.89%. Report is 1 commits behind head on main.

Files	Patch %	Lines
solang-parser/src/helpers/fmt.rs	20.83%	38 Missing :warning:
solang-parser/src/pt.rs	0.00%	6 Missing :warning:
src/sema/ast.rs	78.57%	3 Missing :warning:
src/sema/dotgraphviz.rs	98.71%	1 Missing :warning:
src/sema/mod.rs	98.93%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1596 +/- ## ========================================== + Coverage 87.74% 87.89% +0.15% ========================================== Files 133 133 Lines 64419 64560 +141 ========================================== + Hits 56524 56746 +222 + Misses 7895 7814 -81 ```

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[seanyoung]

Merging for #1560