tree-ops optimizations for large trees

[DrChainsaw]

Some optimizations for large trees and slow disks:

• Rewrite of a couple of tree-ops (mainly regex_rewrite_tree which powers mv and cp) to avoid excessive copying.
• Canged path and Path to drastically reduce the amount of allocations from concatenating strings.
• Add paralleldepth and follow_symlinks to FileTree constructor.

I realized that the pre-walkdir implementation actually followed symlinks by default, and walkdir has follow_symlinks set to false as default which technically made that commit breaking. Defaulting it to true fixes that breakage. Also, contrary to the intuition, follow_symlinks=true is actually the faster option as when set to false it causes walkdir to stat each directory to test if it a symlink (which is not a huge cost since this generally does not touch the filesystem, but still).

mv speedup example:

julia> selfirst(x,y) = x;

julia> t1 = maketree("root" => [string(x) => [string(y) => [(name="data_$(x)_$(y)", value=(x,y))] for y in 1:100] for x in 1:100]);

0.4.3 (after warmup):

julia> length(files(t1))
10000

julia> @time mv(t1, r"(0|13)", s"4"; combine=selfirst);
 21.483043 seconds (150.06 M allocations: 6.289 GiB, 4.15% gc time)

This PR (after warmup):

julia> length(files(t1))
10000

julia> @time mv(t1, r"(0|13)", s"4"; combine=selfirst);
  0.044058 seconds (671.86 k allocations: 24.961 MiB)

FileTree ctor example:

julia> @time FileTree(evilnfsdir);
186.243081 seconds (7.95 M allocations: 701.794 MiB, 0.70% gc time)

julia> @time FileTree(evilnfsdir);
 87.029311 seconds (7.95 M allocations: 701.799 MiB, 1.43% gc time)

julia> @time FileTree(evilnfsdir; paralleldepth=Inf);
 34.147295 seconds (8.18 M allocations: 719.123 MiB, 2.28% gc time, 47 lock conflicts)

julia> @time FileTree(evilnfsdir; paralleldepth=Inf);
 13.578561 seconds (8.18 M allocations: 718.286 MiB, 8.87% gc time, 51 lock conflicts)

@ghyatzo: This might be interesting for you.