Optimize u64_to_{le,be}_bytes

LLVM fails to properly optimize the shifts used to convert the source
value to the right endianess. The resulting assembly copies the value
to the stack one byte at a time even when there's no conversion required
(e.g. u64_to_le_bytes on a little endian machine).

Instead of doing the conversion ourselves using shifts, we can use the
existing intrinsics to perform the endianess conversion and then
transmute the value to get a fixed vector of its bytes.

Before:
````
test be_i8  ... bench:     21442 ns/iter (+/- 70)
test be_i16 ... bench:     21447 ns/iter (+/- 45)
test be_i32 ... bench:     23832 ns/iter (+/- 63)
test be_i64 ... bench:     26887 ns/iter (+/- 267)

test le_i8  ... bench:     21442 ns/iter (+/- 56)
test le_i16 ... bench:     21448 ns/iter (+/- 36)
test le_i32 ... bench:     23825 ns/iter (+/- 153)
test le_i64 ... bench:     26271 ns/iter (+/- 138)
````
After:
````
test be_i8  ... bench:     21438 ns/iter (+/- 10)
test be_i16 ... bench:     21441 ns/iter (+/- 15)
test be_i32 ... bench:     19057 ns/iter (+/- 6)
test be_i64 ... bench:     21439 ns/iter (+/- 34)

test le_i8  ... bench:     21438 ns/iter (+/- 19)
test le_i16 ... bench:     21439 ns/iter (+/- 8)
test le_i32 ... bench:     21439 ns/iter (+/- 19)
test le_i64 ... bench:     21438 ns/iter (+/- 22)
````