MaybeUninint instead of the deprecated mem::uninitialized (given the unstable maybe_uninit_slice feature, in the future this should use slice_assume_init_ref)
Two unittests
serde::*::StdError for our Errors
The bump in serde version further is not needed or helpful AFAICT (this is a library).
The reorganization of the code using MaybeUninit in #59 is not necessary
This takes the remaining bits from https://github.com/rust-embedded-community/serde-json-core/pull/59 by @jordy25519