Bubble up the inner expression

[mishamsk]

Hi,

This mostly implements https://github.com/Fatal1ty/mashumaro/issues/136 (see below)

Caveats:

• The codecs code is pretty dense, I've tried to understand it without the comments, but do not feel confident enough to apply any changes there => the new features may or may not work for codecs
• I dropped the ball on implementing the full path for sequences. I do not think that their going to be any noticeable performance penalty to track current sequence index, but to implement it, the whole unpack_collection must change from an expression generator to a full sub-builder. That's a bit too much for my time constraints;-)

I'd love this to be merged though. I think it adds quite a lot of value already.

Let me know what you think!

[pep8speaks]

Hello @mishamsk! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file mashumaro/core/meta/code/builder.py:

Line 1230:80: E501 line too long (93 > 79 characters) Line 1239:80: E501 line too long (93 > 79 characters)