Dynamically sized return support for external calls

[g-r-a-n-t]

What was wrong?

closes #405
closes #400
closes #321

I also noticed that there was a bug preventing us from calling functions on contract fields (e.g. self.my_contract).

How was it fixed?

Used returndatasize and returndatacopy to deal with sizes that aren't known at compile-time (it's now much simpler).

See the note at the bottom of this section:

The call* instructions use the out and outsize parameters to define an area in memory where the return or failure data is placed. This area is written to depending on how many bytes the called contract returns. If it returns more data, only the first outsize bytes are written. You can access the rest of the data using the returndatacopy opcode. If it returns less data, then the remaining bytes are not touched at all. You need to use the returndatasize opcode to check which part of this memory area contains the return data. The remaining bytes will retain their values as of before the call.

To-Do

• OPTIONAL: Update Spec if applicable

• Add entry to the release notes (may forgo for trivial changes)

• Clean up commit history

[g-r-a-n-t]

Originally, calls to contract fields would fail (e.g. self.foo.get_my_string()). This is because the call was being made to a non-existent contract at the address of the last 20 bytes of the storage pointer.

[cburgdorf]

Adding features by simplifying the code, I like!