Optimize u64_from_be_bytes()

Instead of reading a byte at a time in a loop we hardcode how to read each size.
We also try to do as few reads as possible by reading as big primitive types as
possible. For example if size is eight we do a single read of a u64 value and
if size is seven we read it as [u32|u16|u8].

Timings on a Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz

-- Before --
running 7 tests
test io::extensions::test::test_u64_from_be_bytes ... ok
test io::extensions::bench::u64_from_be_bytes_4_aligned         ... bench:       386 ns/iter (+/- 5)
test io::extensions::bench::u64_from_be_bytes_4_unaligned       ... bench:       387 ns/iter (+/- 2)
test io::extensions::bench::u64_from_be_bytes_7_aligned         ... bench:       628 ns/iter (+/- 1)
test io::extensions::bench::u64_from_be_bytes_7_unaligned       ... bench:       637 ns/iter (+/- 3)
test io::extensions::bench::u64_from_be_bytes_8_aligned         ... bench:       727 ns/iter (+/- 18)
test io::extensions::bench::u64_from_be_bytes_8_unaligned       ... bench:       723 ns/iter (+/- 22)

callgrind rustc -S  rust/src/test/bench/sudoku.rs
    u64_from_be_bytes self: 4.37%

-- After --
running 7 tests
test io::extensions::test::test_u64_from_be_bytes ... ok
test io::extensions::bench::u64_from_be_bytes_4_aligned         ... bench:       162 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_4_unaligned       ... bench:       164 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_7_aligned         ... bench:       201 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_7_unaligned       ... bench:       210 ns/iter (+/- 9)
test io::extensions::bench::u64_from_be_bytes_8_aligned         ... bench:       163 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_8_unaligned       ... bench:       163 ns/iter (+/- 10)

callgrind rustc -S  rust/src/test/bench/sudoku.rs
    u64_from_be_bytes self: 1.78%