Add basic optimizations for allocations in OER

[Nicceboy]

Lack of pre-allocations and not using existing buffers created many unnecessary allocations.

This will reduces them a bit.
I trialed several data types for output buffer, and

output: alloc::borrow::Cow<'a, RefCell<Vec<u8>>>,

seems to be best. I tried sharing direct reference of Vec<u8> and also Bytes/BytesMut.
Direct reference was too difficult to add since encoder uses itself recursively. BytesMut was much slower. Maybe this is okay for a while.

Now the buffer is re-used when possible. e.g. no length calculation requirement afterwards.

Current improvement based on the integer test:

[XAMPPRocky]

Thank you for your PR!