include onchain measurements and benchs

[crypt0miester]

solana programs can already link this crate, since the portable path compiles for SBF with default features. this PR makes that path cheap to carry and puts numbers behind it.

CU per call, mollusk 0.15 on the agave 4.2 runtime, platform-tools v1.54.

	tape	five8	bs58
encode_32	744	1,231	9,520
decode_32	770	1,609	7,658
encode_64	1,741	3,015	35,704
decode_64	2,382	4,589	27,814

for scale, a pubkey encode or decode costs less than half of one create_program_address syscall (1,584 CU). The syscall does no base58, it is only a familiar unit of budget.

notes:

• the variable feature stays off in programs. it overflows the 4 KB SBF stack frame and its tables are 420 KB
• one runtime build measured. CU prices are protocol constants and they move across major Agave releases