Add USDT support.

[tobz]

This PR adds USDT support, based on a transliteration of the Python implementation in iovisor/bcc.

There's a lot of "icing" here as well: cleaning up styling, formatting, condensing code, etc.

Closes #167

[brayniac]

Thanks for working on this. Overall it looks pretty good.

[brayniac]

There's a few other things that clippy is pointing out in this PR. Would be good to address the ones in the new code (src/usdt/mod.rs) - I think I commented on the only new one in core, there's some existing code causing some clippy warnings which can be ignored for the sake of this PR.

[javierhonduco]

Very interested in this! Happy to take a look once it's ready for review! :)

[tobz]

Went through and did a bunch of cleanup and various bits, namely....

• There's now a usdt feature flag, added only by the versions that support the C API functions we required.
• Per the discussion about the error message around converting to CString, I've cleaned up all instances where a CString was being made, and added a contextual error message properly describing the error.
• Moved the usdt module into core so it could follow the file-per-major-API-changes approach.
• Tried my best to collapse redundant code in BPFBuilder.
• Fixed import groupings where applicable.

I realize that the CString stuff might be too noisy when trying to properly review the core aspect of this PR -- USDT support -- so I'm happy to revert those changes if need be.

[brayniac]

Thanks @tobz - this is looking pretty solid now. I'll do one more pass of review once CI is green. Appreciate the contribution.