[foreign-memaccess] layouts, bits and bytes: call for bikeshed :-)

[Maurizio Cimadamore]

Hi,
currently the MemoryLayout API is defined internally in terms of bits; 
this is for a reason, as we want to be able to use layouts for sub-bytes 
entities (e.g. bitfields). That said, the API is quite liberal in 
allowing the user to interact with it either in bits or bytes. For 
instance both layout size and alignment can be retrieved in the 
preferred format.

There's an exception to this rule: layout creation. We have two 
factories, namely:

MemoryLayout::ofValueBits
MemoryLayout::ofPaddingBits

Which do not provide the corresponding XYZByte overloads.

If we were to add such overloads, the next hurdle would be the set of 
constants in MemoryLayouts, where we have e.g.

ValueLayout BITS_8_LE
ValueLayout BITS_16_LE
...

Perhaps it would make sense to expose these in bytes too as well as 
bits, and add:

ValueLayout BYTES_1_LE
ValueLayout BYTES_2_LE
...


How do people feel about these changes? Useful? Confusing?

Let's paint this bikeshed :-)

Cheers
Maurizio