Consider deleting stdvec flavor

[dtolnay]

• postcard::to_stdvec is 100% equivalent to postcard::to_allocvec
• postcard::to_stdvec_cobs is equivalent to postcard::to_allocvec_cobs
• postcard::to_stdvec_crc32 is equivalent to postcard::to_allocvec_crc32
• postcard::ser_flavors::StdVec is exactly the same type as postcard::ser_flavors::AllocVec

On every target platform, Rust compiler, and Cargo feature combination where someone can use a "stdvec" API, the identical "allocvec" API is also available and behaves exactly the same. Having 2 names for the same functionality bloats the API and misleadingly implies that there might be some difference.

It is possible that separating "stdvec" and "allocvec" used to make sense for compilers older than 1.36.0 (2019) where extern crate alloc was not yet stable, but this has not been the case since postcard 0.3.

[jamesmunns]

iirc "to_stdvec" came first, when we didn't have an alloc api (only no_std and std), and I probably should have renamed it then.

I'll clean this up for postcard 2.0.