base16

Make code more bulletproof in the case of panics when using the decode_buf or encode_config_buf functions.

Previously, if the innermost encode/decode function paniced, code that inspected the contents of the Vec (either in a Drop implementation, or by catching the panic) could read from uninitialized memory.

However, I don't believe it is possible for the innermost encode/decode functions to panic, so I don't think there was any risk in previous versions.

I don't believe the panic is possible because only two panics exist in the generated assembly (both only in debug configuration, and not in release). The two panics are respectively:

• a debug_assert verifying the caller performed a check (which it does).

• a usize overflow check on an index variable, which is impossible as we've already tested for that.

That said, this is some powerful rationalization, so I'm cutting a new version with this fix anyway.

Additionally, several functions that previously used unsafe internally now either use less unsafe, or are entirely safe.

encode_byte now returns [u8; 2] instead of (u8, u8), as in practice this tends to be more convenient.

The use of std which requires the alloc trait has been split into the alloc feature.

base16 has been relicensed as CC0-1.0 from dual MIT/Apache-2.0.