feat(init): function support

[mhasel]

Adds a lowering stage in the pipeline to rusty. A new AstVisitorMut trait is introduced, which basically copies the existing AstVisitor trait line for line, but with mutable borrows of visited items instead to allow for in-place manipulation/replacements.

Init functions are now lowered into the AST after the annotation-step. A new AST is created and a new index and annotations are created based on the updated AST.

Adds support for stack-local initializers in POUs (functions, VAR_TEMP)

Out of scope of this PR:

• complex structures (ARRAY[..] OF STRUCT/POU)
• non-default-initializers in init-functions (fb: foo := (s1 := REF(ps)); such that __init_foo initializes foo.s1 with the address of ps instead of the default type initializer)
• exclude lowered/internally generated POUs/statements from generating debug-info

I will create tracking issues for these features once this PR has been reviewed and approved.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.86928% with 124 lines in your changes missing coverage. Please review.

Project coverage is 91.37%. Comparing base (be78df6) to head (7645b17). Report is 53 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/plc_ast/src/mut_visitor.rs	68.43%	107 Missing :warning:
src/lowering.rs	96.43%	15 Missing :warning:
compiler/plc_ast/src/ast.rs	0.00%	1 Missing :warning:
src/lowering/initializers.rs	99.57%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1285 +/- ## ========================================== + Coverage 91.11% 91.37% +0.26% ========================================== Files 153 159 +6 Lines 44816 48469 +3653 ========================================== + Hits 40833 44290 +3457 - Misses 3983 4179 +196 ```

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