Use more refined array type aliases

[ivmarkov]

A follow-up from #334 concerning Case and Pake

• Rather than &[u8] use concrete array types (so that the sizes of these are clear). These arrays store AES-128 keys, Sha256 hashes, fixed-size random values etc. - i.e. their sizes are crystal clear at compile time and we better capture those in the type system
• Reduce a few heapless::Vec arrays to smaller sizes
• Mark with "TODO MEDIUM BUFFER" a few more places where we allocate relatively large-ish values on-stack

[ivmarkov]

@gmarcosb FYI and not ready yet.

[github-actions]

PR #336: Size comparison from 7f5884f to 4170a8f

Full report (8 builds for dimmable-light, onoff-light, onoff-light-bt, rs-matter-core, speaker)

platform	target	config	section	7f5884f	4170a8f	change	% change
dimmable-light	infologs-optz-ltofat	x86_64-unknown-linux-gnu	FLASH	1964272	1963800	-472	-0.0
			RAM	42736	42736	0	0.0
onoff-light	infologs-optz-ltofat	x86_64-unknown-linux-gnu	FLASH	1875312	1874856	-456	-0.0
			RAM	42720	42720	0	0.0
onoff-light-bt	infologs-optz-ltofat	x86_64-unknown-linux-gnu	FLASH	3231720	3231224	-496	-0.0
			RAM	9308	9308	0	0.0
rs-matter-core	infodefmt-optz-ltofat	riscv32imac-unknown-none-elf	FLASH	393022	393368	346	0.1
			RAM	68976	68976	0	0.0
		thumbv6m-none-eabi	FLASH	334368	334220	-148	-0.0
			RAM	65364	65364	0	0.0
		thumbv7em-none-eabi	FLASH	307068	307056	-12	-0.0
			RAM	64856	64856	0	0.0
	infologs-optz-ltofat	x86_64-unknown-linux-gnu	FLASH	863071	863247	176	0.0
			RAM	73650	73650	0	0.0
speaker	infologs-optz-ltofat	x86_64-unknown-linux-gnu	FLASH	1937008	1937256	248	0.0
			RAM	5480	5480	0	0.0