Return empty Vec on ZST

[wcampbell0x2a]

• Return an empty vec when instructed to read to end or certain bytes
  but the struct is a ZST

  NOTE: I tried to solve this using const assertions, but quickly ran into the following. I think you could fix it with unsafe? But not very sure about that solution.

error[E0401]: can't use generic parameters from outer item
  --> src/impls/vec.rs:73:34
   |
72 | const fn assert_not_zst<T>() {
   |                         - type parameter from outer item
73 |     const_assert!(mem::size_of::<T>() > 0);
   |                                  ^ use of generic parameter from outer item
   |
   = note: a `const` is a separate item from the item that contains it

Closes #513

[github-actions]

Benchmark for 075306a

Click to view benchmark

Test	Base	PR	%
count_from_u8_specialize	99.6±0.59ns	99.8±0.81ns	+0.20%
count_no_specialize	1867.9±19.04ns	1848.5±20.38ns	-1.04%
count_specialize	98.5±1.06ns	102.0±1.14ns	+3.55%
deku_read_bits	1230.8±14.11ns	1253.7±16.45ns	+1.86%
deku_read_byte	5.6±0.06ns	5.6±0.04ns	0.00%
deku_read_enum	4.2±0.05ns	4.1±0.03ns	-2.38%
deku_read_vec	30.7±0.36ns	30.6±0.47ns	-0.33%
deku_write_bits	240.4±4.02ns	228.4±4.86ns	-4.99%
deku_write_byte	24.0±0.34ns	24.3±0.34ns	+1.25%
deku_write_enum	22.8±0.35ns	23.1±0.30ns	+1.32%
deku_write_vec	437.9±5.96ns	453.9±4.86ns	+3.65%
read_all	4.4±0.03µs	4.3±0.06µs	-2.27%
read_all_bytes	4.3±0.06µs	4.3±0.06µs	0.00%