# compile
- hmr script is generated with moduleFilename->{index, hash, parents} map (`moduleMeta`).
  this object should be treated as immutable and not changed.
- hmr script added as an entry
- all other entries lose their entry status and get required by the hmr script

# first run
- hmr script runs
- `moduleIndexesToNames` is made as a reverse-lookup table to `moduleMeta`
- `runtimeModuleInfo` is made to track latest `moduleMeta` info and other runtime stuff
- defines global._hmr which defines the module.hot api in all modules
- requires all original entries

# Reload
check
- reloads the script, re-runs it
- new hmr script sees that global._hmr already exists
- global._hmr.newLoad is set to the new moduleDefs, moduleMeta, moduleIndexesToNames objects
- builds a list of outdated modules:
  - get the modules that have new hashes. Include modules that have been added or removed.
  - get the modules that are outdated because they are un-accepting/declining parents of outdated modules:
    - for each outdated module
      - skip if module accepts or declines self, or isn't currently loaded
      - for each parent
        - if parent does not accept or decline module, add to end of outdated module list
- calls the user's callback with the list of outdated modules
apply
- builds a list of outdated modules, same steps as in check
- builds a list of accepted updates:
  - for each outdated module
    - if currently module currently exists
      - if there are any decliners which aren't outdated, then skip
      - if any not-outdated parents and no (accepters or self accepters), then
        skip (should only happen to original entries)
    - add to accepted list
- if ignoreUnaccepted is false and length of accepted updates list != length of
  outdated modules list, then abort
- prepare for execution
  - execute all accepted modules' dispose callbacks, save data to module.hot.data.
    try-catch this and fail apply if error.
  - remove all accepted modules' exports
  - remove all accepted modules from parent lists
  - update runtimeModuleInfo and moduleDefs to match new
- for each accepted module, if it self-accepted, require it, otherwise call its
  parents' accepters.
  try-catch this. If module accepted self, give it the error and continue,
  otherwise keep the error, continue, and deliver it to callback at end.