feat: add alloc feature

[pmnoxx]

Rust is built around the concept of “zero cost abstraction”.

It's best not to hide extra allocation costs. I see, that by default there is an implementation available for HashSet and HashMap. It's a good idea to put it behind use_alloc feature.

In order not to break backwards compatibility, I added it to default_features.

[matklad]

borsh already supports no_std, and this feature works correctly. On no-std, HasMap and HashSet are hashbrown variations. This itself is somewhat questionable design descisions though.

Note that HashMap isn't actually a part of alloc crate, as it uses access to randomness, which is not a part of alloc.

[matklad]

This changes a public API, this shouldn't be done lightly. See #51 for the context on things which could be better, but which we can't just do, as that'll break the world.

[pmnoxx]

borsh already supports no_std, and this feature works correctly. On no-std, HasMap and HashSet are hashbrown variations. This itself is somewhat questionable design descisions though.

Note that HashMap isn't actually a part of alloc crate, as it uses access to randomness, which is not a part of alloc.

FYI, borsh can't be compiled without std. @matklad

> cargo build --no-default-features

error: cannot find macro `vec` in this scope
   --> borsh/src/de/mod.rs:283:30
    |
283 |             let mut result = vec![T::deserialize(buf)?];
    |                              ^^^
    |
    = note: consider importing one of these items:
            alloc::vec
            crate::maybestd::vec

error: could not compile `borsh` due to previous error