Migrate ASCII acceleration code to align_to/align_to_mut

[hsivonen]

Currently, the ASCII acceleration code manually reinterprets slice memory as wider SIMD or ALU types. This code predates the align_to and align_to_mut methods on slices.

This code should be rewritten to use these methods with the middle slice being a SIMD type or a wider ALU type in the aligned case or a fixed-length array that can be unalignedly read as a SIMD type for unaligned SIMD.