System image compression with zstd

[xal-0]

Revived version of #48244, with a slightly different approach. This version looks for a function pointer called jl_image_unpack inside compiled system images and invokes it to get the jl_image_buf_t struct. Two implementations, jl_image_unpack_zstd and jl_image_unpack_uncomp are provided (for comparison). The zstd compression is applied only to the heap image, and not the compiled code, since that can be shared across Julia processes.

TODO: test a few different compression settings and enable by default.

Example data from un-trimmed juliac "hello world":

156M  hello-uncomp
 43M  hello-zstd
 48M  hello-zstd-1
 45M  hello-zstd-5
 43M  hello-zstd-15
 39M  hello-zstd-22

$ hyperfine -w3 ./hello-uncomp 
Benchmark 1: ./hello-uncomp
  Time (mean ± σ):      74.4 ms ±   0.8 ms    [User: 51.9 ms, System: 19.0 ms]
  Range (min … max):    73.0 ms …  76.6 ms    39 runs

$ hyperfine -w3 ./hello-zstd-1
Benchmark 1: ./hello-zstd-1
  Time (mean ± σ):     152.4 ms ±   0.5 ms    [User: 138.2 ms, System: 12.0 ms]
  Range (min … max):   151.4 ms … 153.2 ms    19 runs
 
$ hyperfine -w3 ./hello-zstd-5 
Benchmark 1: ./hello-zstd-5
  Time (mean ± σ):     154.3 ms ±   0.5 ms    [User: 139.6 ms, System: 12.4 ms]
  Range (min … max):   153.5 ms … 155.2 ms    19 runs

$ hyperfine -w3 ./hello-zstd-15
Benchmark 1: ./hello-zstd-15
  Time (mean ± σ):     135.9 ms ±   0.5 ms    [User: 121.6 ms, System: 12.0 ms]
  Range (min … max):   135.1 ms … 136.5 ms    21 runs
 
$ hyperfine -w3 ./hello-zstd-22
Benchmark 1: ./hello-zstd-22
  Time (mean ± σ):     149.0 ms ±   0.6 ms    [User: 134.7 ms, System: 12.1 ms]
  Range (min … max):   147.7 ms … 150.4 ms    19 runs

[gbaraldi]

We probably want to mmap this with huge pages/large pages