Extract alloc feature (2.0 candidate)

[dj8yfo]

While working on an embedded app, it was discovered, that it's not possible to use borsh crate when global_allocator is not defined.
One possible solution in such cases is to define global_allocator , another is to extract alloc feature and to use borsh with default_features = false .

std = ["alloc"]
# Provide impls for types in the Rust core allocation and collections library
# including String, Box<T>, Vec<T>, and Cow<T>. This is a subset of std but may
# be enabled without depending on all of std.
alloc = []

Below is superficial description of changes required:

• extract everything about alloc crate into separate feature
  • change borsh/src/nostd_io.rs or make 2 variants, depending on alloc feature
  • make BorshDeserialize trait method vec_from_reader depending on alloc feature
  • each usage of std::io::Error::new and borsh::io::Error::new implies depending on alloc feature, there's around 50 occurencies of it in borsh
  • no change required to BorshSerialize
  • make unstable__schema feature imply alloc feature

// from
enum Repr {
    Simple(ErrorKind),
    Custom(Custom),
}

impl Error {
    pub fn new<T: Into<String>>(kind: ErrorKind, error: T) -> Error {
        Self::_new(kind, error.into())
    }

    fn _new(kind: ErrorKind, error: String) -> Error {
        Error {
            repr: Repr::Custom(Custom { kind, error }),
        }
    }
}

// to
enum Repr {
    Simple(ErrorKind),
}
impl Error {
  
}

For reference an example of replicating a subset of borsh without dependency on alloc crate is provided.

[dj8yfo]

original motivation to create this issue was kind of resolved by LedgerHQ/ledger-device-rust-sdk#155, but the heap on the device was still not quite usable on the device with smallest capabilities, as mentioned in
comment , EDIT: support of which (smaller devices) was removed later

With the mentioned tiniest target platform being a concrete example of environment imposing specific requirements, most likely an endeavor to resolve this issue will only make sense together with changing the signature of BorshDeserialize:

fn deserialize(...) -> Result<Self>;
to 
fn deserialize(..., &mut self) -> Result<()>;