also revise an `unsafe` that might be unsafe un extremely unlikely
circumstances - no one should be passing yaxpeax a `&[u8]` larger than
`isize::MAX`, but on 32-bit architectures we can't necessarily guarantee
that it won't happen

the previous test and code only tested one concrete archtecture, and it
turns out never required `std::error::Error` on `DecodeError`.

included is mandataing a `description` method on `DecodeError`
implementors - already approximately required by the Debug and Display
anyway.

also include a StandardPartialDecoderError for incomplete decoders to
use. i expect that one of the last steps of a decoder's 1.0 release will
be to move away from this.

this is useful for instruction sets like arm where instructions are
guaranteed to be 4 bytes, as well as instruction sets where an
instruction word is not a multiple of u8 bytes.

additional variants will require clients to implement DecodeError, still