This reverts commit ba2f6972193cdbd7c1e77e26212513e47926b115.

Box already used ruby_api_version_name.

This reverts commit 9b576cd6255aba97e5e2f55f4b09f00c7dd0e839.

Re-open the exising module by calling `rb_define_module`.
RDoc (`RDoc::Parser::C#do_classes_and_modules`) does not recognize
`rb_path2class` as a class/module definition.

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 (Shopify/ruby#667)

Left for future enhancements:

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

Closes Shopify#854

Previously, when `./configure --enable-yjit=dev --enable-zjit` JIT_DESCRIPTION was
defined to be "ZJIT", but when running with `--yjit`, ruby_set_yjit_description()
calls `define_ruby_description("+YJIT dev")`, which is longer than the
following assertion expects:

    RUBY_ASSERT(n <= ruby_description_opt_point + (int)rb_strlen_lit(JIT_DESCRIPTION));

Use the concatenation of the two strings -- the contents of the string
is never used. Minimize the scope of the macro, too.