Unified strategy for handling alignment

[joshlf]

This issue tracks developing a unified strategy for handling various alignment concerns that may interact with one another.

Currently:

• Add aligned byteorder types #254
• Add Align type #249
• Extend Unalign to support arbitrary alignment #205
• Support eliding alignment check when reading from aligned buffer #280
• Feature proposal: Layout gadgets. #381
• Encode size and alignment in the type system #1316