Add serialization helpers to improve api ergonomics

[jstarry]

Fixes: #31

Problem

BorshSerialize::serialize method signature is not very ergonomic when using mutable slices. This is because a &mut [T] already implements Write but the method expects a &mut W where W itself implements Write. This means that in order to pass a mutable slice, you need to pass a mutable reference to the mutable slice: &mut &mut [T].

Changes

• Added top-level serialize_to_writer and serialize_to_vec helper functions

[matklad]

needs a test & different argument order, but otherwise LGTM.

I'd probably use to_vec, to_writer names (short & follows serde_json), but I am ok with these names as well.

[ailisp]

Looks good to me. Thank you!