refactor: modularize files and refactor existing implementations

[alejandro-vaz]

this PR is the preliminary implementation of #427

scope of this PR

this PR aims to refactor the codebase to make it navigable

it starts to split the old 3k LOC lib.rs into small submodules based on abstraction boundaries:

• allocationerror.rs => error types
• bytes.rs => functionality provided by the bytes crate, gated by a cfg
• comparisons.rs => Eq and Ord, total and partial
• lib.rs => remaining not-yet-refactored code
• mallocsizeof.rs => MallocSizeOf work gated behind a cfg
• rawsmallvec.rs => already implemented by @zachs18 on Make RawSmallVec (and its fields) public only under feature = "internals". (v2) #348
• references.rs => asref, borrow, deref. for const and mut references
• serde.rs => serde's serialize and deserialize gated behind a cfg
• std.rs => standard library implementations. currently, only std::io::Write
• taggedlen.rs => TaggedLen and its implementation
• tests.rs => tests, already present before

other changes

• renamed CollectionAllocErr to AllocationError
• removed an is_zst<T>() helper which can be inlined by a simple comparison
• changes rustfmt.toml a bit to group imports and removed the use of the vec! macro so the formatter didn't get confused about whether it was referencing the vec module or the vec! macro

Note

documentation doesn't build properly, see #439

Note

experimented a bit with rustfmt.toml, should make files easier to read now

[zachs18]

I don't think removing all blank lines improves readability; I think it makes it worse. Also note that servo/servo doesn't do this (example)

[alejandro-vaz]

fair

then probably 0 for lower bound and 1 for upper I guess. 2+ looks weird to me

did it out of inertia, didn't think much about it, I'll fix it tomorrow

code looks weird though now that you're saying it