XML deserializer & parser

[mhasel]

In order to make the code-review process more easily digestible, the CFC feature PR has been split into multiple parts. At first, only AST elements that are compatible with existing Structured Text - AST are introduced.

In this PR:

• XML peekable reader
• deserializer & serializer
• model-to-AST transformer for assignments and call-statements

Co-authored by @volsa

[codecov]

Codecov Report

Patch coverage: 82.52% and project coverage change: -0.40% ⚠️

Comparison is base (f7fbc52) 95.96% compared to head (029f6fe) 95.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #883      +/-   ##
==========================================
- Coverage   95.96%   95.56%   -0.40%     
==========================================
  Files         128      150      +22     
  Lines       38781    39969    +1188     
==========================================
+ Hits        37217    38198     +981     
- Misses       1564     1771     +207     

Files Changed	Coverage Δ
compiler/plc_derive/src/lib.rs	100.00% <ø> (ø)
compiler/plc_derive/tests/rusty_derive_tests.rs	100.00% <ø> (ø)
compiler/plc_xml/src/model/action.rs	0.00% <0.00%> (ø)
compiler/plc_xml/src/model/connector.rs	0.00% <0.00%> (ø)
compiler/plc_xml/src/model/control.rs	0.00% <0.00%> (ø)
compiler/plc_xml/src/xml_parser/action.rs	0.00% <0.00%> (ø)
src/lexer.rs	97.57% <ø> (ø)
src/parser.rs	97.48% <ø> (ø)
src/validation.rs	94.93% <ø> (ø)
compiler/plc_xml/src/error.rs	29.16% <29.16%> (ø)
... and 20 more

... and 2 files with indirect coverage changes

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[ghaith]

Could we add this behind a feature flag for now? (fbd or cfc)