Represent kinds in symbolic form in ASR

Currently the AST->ASR phase evaluates all kinds as integer and stores them as integers in ASR.

That requires to have the whole evaluation machinery (with all intrinsic functions) in ast_to_asr.cpp, and it gets quite messy, as seen in !900 (merged).

Also, we need to be able to represent parametrized derived types (PDT), where the kind is stored symbolically (like a C++ template) and only determined at "instantiation time".

For these reasons, I think it is cleaner to decouple and in AST->ASR simply keep all kinds in symbolic form, i.e., keep real(dp) as is.

Then have a pass that evaluates all kinds and substitutes real(dp) for real(8).