First we add an optional API parameter for `sizehint!` that controls
whether it is for `push!` (default) or `pushfirst!`. Secondly, we make
the offset zero when using `sizehint!` to shrink an array from the end,
or the trailing size zero when using it to shring from the beginning.

Then we replace the prior implementations of `prepend!` and `append!`
with ones that are safe even if the iterator changes length during the
operation or if convert fails. The result of `prepend!` may be in an
undefined order (because of the `reverse!` call) in the presence of
concurrent modifications or errors, but at least all of the elements
will be present and valid afterwards.

Replaces and closes #49905
Replaces and closes #47391
Fixes #15868

Benchmarks show that repeated `push!` performance (with sizehint) is
nearly equivalent to the old append performance:
```
julia> @benchmark append!(x, 1:1000) setup=x=Vector{Float64}(undef,0)
BenchmarkTools.Trial: 10000 samples with 10 evaluations.
 Range (min … max):  1.027 μs … 72.871 μs  ┊ GC (min … max): 0.00% … 94.57%
 Time  (median):     1.465 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):   1.663 μs ±  1.832 μs  ┊ GC (mean ± σ):  6.20% ±  5.67%

             ▂▃▅▆█▇▇▆▄▂▁                                      
  ▂▁▁▂▂▂▂▃▄▅▇█████████████▇▆▅▅▅▅▅▅▄▅▄▅▅▅▆▇███▆▅▄▃▃▂▂▂▂▂▂▂▂▂▂ ▄
  1.03 μs        Histogram: frequency by time        2.31 μs <

 Memory estimate: 19.69 KiB, allocs estimate: 0.

julia> @benchmark append!(x, 1:1000) setup=x=Vector{Int}(undef,0)
BenchmarkTools.Trial: 10000 samples with 10 evaluations.
 Range (min … max):  851.900 ns … 76.757 μs  ┊ GC (min … max): 0.00% … 91.59%
 Time  (median):       1.181 μs              ┊ GC (median):    0.00%
 Time  (mean ± σ):     1.543 μs ±  1.972 μs  ┊ GC (mean ± σ):  6.75% ±  5.75%

     ▆█▇▃                                                       
  ▂▃██████▇▅▅▄▅▅▃▂▂▂▃▃▃▂▃▃▃▂▂▂▂▂▁▂▁▂▁▂▂▂▁▁▂▂▁▁▁▁▁▁▁▂▂▂▃▃▃▃▂▂▂▂ ▃
  852 ns          Histogram: frequency by time         4.07 μs <

 Memory estimate: 19.69 KiB, allocs estimate: 0.
```

Co-authored-by: Sukera <Seelengrab@users.noreply.github.com>
Co-authored-by: MasonProtter <mason.protter@icloud.com>