Considering removing `from_slice` from newtype APIs

[brycx]

Since the introduction of serde support, most newtypes implement TryFrom<&[u8]> which simply delegates to from_slice(). This seems a bit redundant, especially if/when moving to Rust 2021 Edition, where TryFrom/TryInto are part of prelude. We could consider removing from_slice in the future and simply provide TryFrom only instead.

[brycx]

See also #237