Add push_reserved() and optimize extend() to avoid unnecessary capacity checks

[SOF3]

Implements the optimization as proposed in #58, renaming the proposed method from push_unchecked to push_reserved for clarity.

Benchmarks

Benchmarks of <ThinVec as From<[u16; N]>>::from(array) show the following improvements.

Environment: Linux 6.16.7-arch1-1, rustc 1.91.0-beta.4, x86_64-unknown-linux-gnu, Intel(R) Core(TM) i7-10510U CPU @ 1.80GHz

N = 13:

criterion:

iai:

bench_thinvec_from_array_and_drop_large
  Instructions:                 331 (-37.66478%)
  L1 Accesses:                  426 (-38.61671%)
  L2 Accesses:                    3 (+50.00000%)
  RAM Accesses:                  14 (-22.22222%)
  Estimated Cycles:             931 (-30.20990%)

N = 1027:

criterion:

iai:

bench_thinvec_from_array_and_drop_huge
  Instructions:                 782 (-94.47896%)
  L1 Accesses:                 1063 (-94.26645%)
  L2 Accesses:                    7 (-80.00000%)
  RAM Accesses:                  90 (-4.255319%)
  Estimated Cycles:            4248 (-80.69530%)

[emilio]

Two minor nits, but looks good.

[emilio]

Let's at least debug_assert!(old_len < self.capacity()); for good measure.

[emilio]

Probably put it inside the hint > 0 for consistency?

[emilio]

Merged it with some nits and going back to push_unchecked since I think it's more consistent with the _unchecked functions in std Vec.