Add ability to append to `abi.DynamicArray`

[jasonpaulos]

This is a draft PR that partially implements support for appending to dynamic arrays.

This has not been tested and isn't fully functional.

[barnjamin]

Would it make sense to have some other types that were better for this type of thing that could be initialized from/encoded into an ABI type?

ex:

StringArray:
  positions: uint64[] (itob'd/concat'd list of string positions, stored in a scratch var)
  elements: string[] (concat'd list of elements with just the byte contents stored in scratch)

str_arr[3] => Extract(ExtractUint64(positions.load(), 8*3), ExtractUint64(positions.load(), 8*(3+1)))

or maybe storing lengths would be better in case some string is replaced, but would need to sum along the way to find the right position.

This would alleviate the need to update each element in the header describing the offset any time you append to the array but I've not tried to write it to see how expensive it is in opcode terms

[FrankSzendzielarz]

There could be a couple of representations of complex types depending on if the element is declared as readonly. Alternatively we could have a representation that works for both scenarios (eg a box that treats all complex type elements as references) and decode/encode only on entry/exit to/from ABI methods. Arbitrary representations might complicate debugger experiences.

In general I feel that the AVM would probably benefit from supporting types like nested tuples and arrays directly, using some kind of element access syntax, and having the AVM take care of the encoding/decoding.