API alignment

In the light of aiming for a stabilized API of v1.0 (#410) but having just done a significant API changeset in 0.8, I wanted to bring something up that has been bugging me several times.

Currently the heapless::Vec API is significantly different from both std::Vec and alternative no_std/no-alloc Vec implementations like tinyvec::ArrayVec or arrayvec::ArrayVec or coca. This is especially true for the important case where std::Vec would reallocate but its fixed size alternatives must either panic or become fallible: e.g. Vec::push().

heapless deviates from the others in its API and makes these fallible. The others are panicing and additionally/alternatively tend to offer a fallible API (ArrayVec::try_push()).

IMO it would be nice if we could overall

• align the API with the std::Vec API and tinyvec
• offer a fallible API (try_push() etc).

That would make porting a bit easier. And it could allow heapless::Vec to be a drop-in replacement for tinyvec::ArrayVec. The only (AFAICT) remaining difference would be under the hood: the use of MaybeUninit.

The API difference comes up from time tom time e.g. #440.
There is demand for distinct operations that are both infallible and non-panicing: #341

There may also be other places in heapless where the API choice diverges from the std analogue.

Start by comparing the heapless::Vec API with std::Vec, tinyvec::ArrayVec, and arrayvec::ArrayVec, then read the discussion in issues #410, #440, and #341. Done means the project has an agreed v1.0 API direction covering std-like operations and fallible alternatives such as try_push().