ZJIT: add support for lazy RubyVM::ZJIT.enable

[chancancode]

This implements Shopify#854:

• Splits boot-time and enable-time initialization, tracks progress with InitializationState enum

• Introduces RubyVM::ZJIT.enable Ruby method for enabling the JIT lazily, if not already enabled

• Introduces --zjit-disable flag, which can be used alongside the other --zjit-* flags but prevents enabling the JIT at boot time

• Adds ZJIT infra to support JIT hooks, but this is not currently exercised (ZJIT: Allow ZJIT to use #with_jit hooks Shopify/ruby#667)

Left for future enhancements:

• Support kwargs for overriding the CLI flags in RubyVM::ZJIT.enable

Closes Shopify#854

[chancancode]

Happy to fill this in here, but not sure what is the approach we want to use here.

The yjit equivalent does this:

let yjit = rb_const_get(rb_cRubyVM, rust_str_to_id("YJIT"));
rb_funcall(yjit, rust_str_to_id("call_jit_hooks"), 0);

We have rb_funcallv in zjit, I suppose I could bind rb_cRubyVM and rb_const_get and make it work, just want to get someone else's opinion before I do that.

Alternatively, since this code will remain dormant anyway until Shopify#667 is resolved, we could also punt for now.

[k0kubun]

We have rb_funcallv in zjit, I suppose I could bind rb_cRubyVM and rb_const_get and make it work, just want to get someone else's opinion before I do that.

Yeah, I think that's what we want to do. I don't come up with any better way.

[k0kubun]

Looks good to me. Thanks!

[chancancode]

Support kwargs for overriding the CLI flags in RubyVM::ZJIT.enable

Shopify#923