Add some conversion options for alloc vecs

[rilipco]

This PR introudces a new feature alloc which will make heapless depend on the alloc crate and adds two trait implementations on heapless::Vec and alloc::vec::Vec to allow conversion between them.

Conversion from heapless::Vec to alloc::vec::Vec is fallible (due to OOM errors) and implemented via TryFrom.
Conversion from alloc::vec::Vec to heapless::Vec is fallible (due to capacity limit) and implemented via TryFrom.

[newAM]

Conversion from heapless::Vec to alloc::vec::Vec is infallible and implemented via From.

On embedded platforms people may want to handle allocation errors locally, such as using try_with_capacity (sadly not stable at the moment).

I'm hesitant to implement an infallible implementation because in #381 I found that implementing infallible conversions prevents fallible ones from being implemented later.

[newAM]

Would you be willing to remove the conversion from heapless::Vec to alloc::vec::Vec for now?

I'm happy to merge from heapless::Vec to alloc::vec::Vec!

[newAM]

I realized a fallible heapless::Vec to alloc::vec::Vec is possible with the current APIs.

alloc::vec::Vec::new will not allocate until elements are pushed onto it. That plus try_reserve should allow for a fallible conversion implementation.