Allow tree shaking of the core environment when it is unobservable.

[andrewchambers]

We can avoid loading the full core environment if the mutable
parts of core are not reachable from the main function. This
is quite a large number of programs.

This change makes 'hello world' 2-3 times faster, and uses approximately
10 times less ram.

[andrewchambers]

I feel this approach could be improved/altered with any of:

• A jpm option for this.
• Tweaks to the C code in peg, such that it does not take things directly from the root-env.

[bakpakin]

I like this idea a lot, and rather than implement hacks to get around certain issues, I think it might be worth considering changing the core library to avoid any calls (directly or indirectly) janet_core_env in all cfunctions.

[bakpakin]

BTW, this draft is not dead, just pending changes to the thread module, which seem to be the biggest hurdle in adding the functionality. , It is currently quite difficult to remove any references to the janet_core_env function in the thread module.

[bakpakin]

The commits I have added have taken some of the code from here, but reworked. Mainly, we don't use the auto-dectection since there are cases where the auto-detection can fail, say when loading a native module that tries to use janet_dobytes at module load time. Other than that, it seems to be working well, and definitely cuts down start time for hello, world!

Example usage:

# hello.janet
(defn main [&] (print "hello, world!"))

With core library disabled:

$ jpm --no-core quickbin hello.janet hello
generating executable c source...
compiling hello.c to build/hello.o...
linking hello...
$ time ./hello
hello, world!

real	0m0.001s
user	0m0.001s
sys	0m0.000s

Without core library disabled:

$ jpm quickbin hello.janet hello2
generating executable c source...
compiling hello2.c to build/hello2.o...
linking hello2...
$ time ./hello2
hello, world!

real	0m0.003s
user	0m0.003s
sys	0m0.000s

[sogaiu]

Here are some local results:

$ jpm --no-core quickbin hello.janet hello
generating executable c source...
compiling hello.c to build/hello.o...
linking hello...

$ multitime -n10 -s1 ./hello
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
===> multitime results
1: ./hello
            Mean        Std.Dev.    Min         Median      Max
real        0.003       0.002       0.001       0.003       0.006       
user        0.001       0.001       0.000       0.001       0.003       
sys         0.002       0.002       0.000       0.002       0.006       

$ jpm quickbin hello.janet hello2
generating executable c source...
compiling hello2.c to build/hello2.o...
linking hello2...

$ multitime -n10 -s1 ./hello2
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
hello, world!
===> multitime results
1: ./hello2
            Mean        Std.Dev.    Min         Median      Max
real        0.008       0.007       0.002       0.004       0.020       
user        0.006       0.006       0.000       0.003       0.016       
sys         0.002       0.002       0.000       0.003       0.004       

[andrewchambers]

nice, thanks, it should be useful for programs like jfmt.