Ranges and for loops

[MichalMarsalek]

Big refactor of integer ranges and loops. The main motivation is to simplify the AST, reduce repetition and special casing in plugins and emitters and to simplify the control flow analysis in the future.

• Closes #33
• Closes #127
• Renames concat alias to + (.. is understood, but emits deprecation warning (this was kind of tricky to implement since it needs type info))
• Removes ForRange, ForDifferenceRange nodes
• Adds range_incl (alias ..), range_excl (alias ..<) as frontend and range_diff_excl as backend opcodes to fill in the functionality
• Adds text_to_list[Ascii], (alias text_to_list) text_to_list[byte], text_to_list[codepoint] opcodes, to be used for iterating over characters.
• Removes ForEachKey & ForEachPair. They were not used and I'm not sure target langs would agree on iteration order. If we decide to add this functionality later, we can do so via a table_keys_list opcode over which we would iterate using a ForEach = for.
• Adds default values for opcodes. These are used when parsing Polygolf and encountering less arguments than the nominal arity and also by mapOps plugin, if the target lang shares the defaults with Polygolf.
• Adds Cast node. This node is used when explicit conversion from/to a type that doesn't exist in Polygolf is needed in the target language.

Polygolf's for now means a for each loop over list items and it should be used for iterating over a range as well, similar to Python. for $i $a $b {}; should now be written for $i ($a ..< $b) {}. Old form is understood (as implicit range_excl), but emits a deprecation warning. for $i 10 {}; and for 10 {}; forms are preserved without a warning. for $i $text {}; is a syntax sugar for for $i (text_to_list $text) {};, for[byte] $i $text {}; is a syntax sugar for for $i (text_to_list[byte] $text) {}; similar for[codepoint].

[Steffan153]

I think several of the language-specific tests need updated, as they still have the old for $i $a $b in them