Add borrow capability constant to Reader

[cpubot]

This adds a constant BORROW_KINDS to the Reader trait that allows implementations to specify which BorrowKinds they support. This will allow SchemaRead implementations to vary their implementation depending on the borrow capabilities of the Reader. Conditions based on Reader::supports_borrow can be constant-folded by the compiler, and as such should have no runtime cost.

Examples of where this is useful

Cow

We don't currently support Cow, but with this functionality, we can return Cow::Borrowed for Readers that support BorrowKind::Backing.

solana-sdk's v1::Message

https://github.com/anza-xyz/solana-sdk/blob/735c03b78c60d7ae9a98c0cdccac396911881676/message/src/versions/v1/message.rs#L651-L658

If we wanted to make this SchemaRead implementation compatible with a broader set of Reader back-ends in the future, this implementation could heap-allocate the InstructionHeaders if the Reader does not support BorrowKind::Backing.

ecow

https://github.com/tanmay4l/wincode/blob/a08c2349a7e2e8ba835602de5b818c5f6c11f7b1/wincode/src/schema/external/ecow.rs#L36-L44

Similarly, this implementation could check reader.supports_borrow(BorrowKind::CallSite) rather than matching on the error.

[kskalski]

I don't like _CAP suffix here, because it sounds like a limit (from "hat") or capacity

How about calling it BORROW_SUPPORT, maybe BORROW_KINDS or just a full BORROW_CAPABILITY ?
Also, given that this is mask, could mention it in the name like BORROW_SUPPORT_MASK

[cpubot]

Sure -- BORROW_KINDS seems reasonable to me

[kskalski]

can we make it const?

[cpubot]

Unfortunately not -- const functions in traits isn't yet supported. But, I looked at the assembly in a few examples, and the compiler reliably constant-folds conditions out since BORROW_CAP and kind.mask is const