Constants validation (fb instances not allowed #295, scoped const expressions #292)

[riederm]

1. fb-instances and class-instances cannot be declared
   as constants.

the implementation does not allow to declare a const
of type FB or Class, an array of FBs or Classes
or a struct that contains a field of type FB or
Class.

Furthermore the DataTypeInformation::Struct now contains
a new field: 'source' that indicates where this struct
comes from. It is either an original Declaration
(TYPE x : STRUCT ... END_TYPE) or it was generated
from a POU(PouType). With this field it is now very
easy to see what a DataType offers (is it an FB or Class,
can I call it, etc.)

closes #295

2. add scope to unresolved const expressions

the additional scope parameter in const_evaluator::evaluate(...)
helps the evaluator to distinguish between global constants and
POU-local ones. When resolving const expressions the given
scope will be used to resolve references.

therefore every ConstExpression holds an optional scope when it
is registered (see enum ConstExpression).

closes #292

[codecov]

Codecov Report

Merging #306 (a369de0) into master (5f2dbcd) will decrease coverage by 0.08%.
The diff coverage is 86.48%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   95.01%   94.92%   -0.09%     
==========================================
  Files          45       45              
  Lines       12928    13034     +106     
==========================================
+ Hits        12283    12372      +89     
- Misses        645      662      +17     

Impacted Files	Coverage Δ
src/validation/variable_validator.rs	89.24% <65.30%> (-10.76%)	⬇️
src/index.rs	92.40% <90.00%> (+0.04%)	⬆️
src/index/const_expressions.rs	94.95% <94.11%> (+1.01%)	⬆️
src/codegen/generators/struct_generator.rs	88.52% <100.00%> (+0.09%)	⬆️
src/index/visitor.rs	96.14% <100.00%> (+0.20%)	⬆️
src/lib.rs	97.33% <100.00%> (+0.04%)	⬆️
src/resolver/const_evaluator.rs	89.53% <100.00%> (+0.19%)	⬆️
src/typesystem.rs	97.01% <100.00%> (+<0.01%)	⬆️

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