sha1: pass block slices to sha1_asm + remove cfg toggle for constants

[tlercher]

This doesn't really depend on RustCrypto/asm-hashes#38 but it is required to make it work for me on a M1 Mac.

As the title says i changed the (internal) API so that block slices instead of independent blocks are passes to sha1_asm.

I also removed the cfg toggles for aarch64, because they are required for the software-fallback and asm-aarch64 is marked deprecated anyway.

Benchmarks on my M1 MacBookPro:

Soft

test bench1_10    ... bench:          14 ns/iter (+/- 0) = 714 MB/s
test bench2_100   ... bench:         120 ns/iter (+/- 1) = 833 MB/s
test bench3_1000  ... bench:       1,151 ns/iter (+/- 4) = 868 MB/s
test bench4_10000 ... bench:      11,465 ns/iter (+/- 130) = 872 MB/s

ASM

test bench1_10    ... bench:           7 ns/iter (+/- 0) = 1428 MB/s
test bench2_100   ... bench:          45 ns/iter (+/- 1) = 2222 MB/s
test bench3_1000  ... bench:         459 ns/iter (+/- 9) = 2178 MB/s
test bench4_10000 ... bench:       4,579 ns/iter (+/- 84) = 2183 MB/s

[tarcieri]

Oh whoops, didn't see this in time, but the same changes were made in #289. Thanks anyway!