Experimental BTF support

[vadorovsky]

Provide a CLI option --btf which provides an experimental BTF support.

This includes a sanitization logic, which modifies the debug info in order to make it acceptable by the kernel, which includes:

• A requirement for names to be compatible with C.
• Removing DI for data-carrying enums.
• Removing name from types annotated with AyaBtfMapMarker - Linux kernel requires maps to be anonymous structures, but Rust doesn't support anonymous types. This is our way to make it working.
• Sorting type members by offsets (sometimes they arrive out of order).
• Setting linkage of all functions which are not BPF programs to static.

Updates: #1 Closes: #177 #26

[vadorovsky]

@dave-tucker I added a small section in README. Do you think it's enough? I would rather document more details (like C-shims) in the book.