add sizehint! for first and make append!/prepend! safer

[vtjnash]

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.

[vtjnash]

@nanosoldier runbenchmarks("array", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.