Make unsound code opt-in.

[finnbear]

Fixes #50

Benchmarked on i7-7700k

Before (default features) or After (unsound feature):
test str::tests2::bench_str_vec_encode                      ... bench:      98,474.54 ns/iter (+/- 34,320.73)
test derive::option::tests2::bench_option_u16_vec_encode    ... bench:         674.79 ns/iter (+/- 102.70)
test bool::test2::bench_bool_vecs_encode                    ... bench:         327.71 ns/iter (+/- 9.80)
test derive::vec::test::bench_vec_encode                    ... bench:          35.98 ns/iter (+/- 2.39)

After (default features):
test str::tests2::bench_str_vec_encode                      ... bench:     126,276.70 ns/iter (+/- 4,599.54)
test derive::option::tests2::bench_option_u16_vec_encode    ... bench:         767.94 ns/iter (+/- 26.88)
test bool::test2::bench_bool_vecs_encode                    ... bench:         454.65 ns/iter (+/- 18.57)
test derive::vec::test::bench_vec_encode                    ... bench:          38.67 ns/iter (+/- 4.02)