Some Macros for Subclassing

[glyh]

Here's a snippt of the macros on GIST. Ideas are based on #228 .

(I've included some tests at the end)

As we've mentioned, this sugar is useful because many APIs requires user to implement subclasses of abstract classes.

[glyh]

Some other examples:

• Indent parser in Lark, reference on documentation.

[yglukhov]

@glyh good job! I would not call it canonical syntax by no means, but it's definitely quite an improvement over my variant :). How about making a separate nimble package (nimpy_inheritance?) and I'll refer it in the README?