Forbid use of as_slice when unaligned feature is enabled

[as-com]

Somewhat related to #496 - if the user enables the unaligned feature, undefined behavior may result from use of as_slice() if the type is larger than 1 byte.

I added a compile-time check that panics in a const fn for a more useful error message and to keep the API the same. This also adds support for as_slice() on big-endian targets so long as the lists' elements are 1 byte or less.

Possible alternatives include:

• Returning None if the buffer is unaligned, but still returning a slice if it happens to be aligned
• Always returning None if the unaligned feature is enabled for lists of multi-byte elements
• #[cfg(...)]-gating the entire method

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 51.69%. Comparing base (ab342b3) to head (9522881). Report is 4 commits behind head on master.

Files	Patch %	Lines
capnp/src/primitive_list.rs	16.66%	10 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #497 +/- ## ========================================== + Coverage 51.64% 51.69% +0.04% ========================================== Files 69 69 Lines 33735 33790 +55 ========================================== + Hits 17422 17467 +45 - Misses 16313 16323 +10 ```

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[dwrensha]

Thanks!