* merlin document support for PPXs (#167)

* initial working with tests

* temp update

* temp

* major refactors, add support for .mli files

* refactoring and cleanup

* move all logic into override_document.ml; cleanup code; add logging

* add many more tests; handle edge cases

* revert std.ml changes

* improve comments

* update more comments

* suggested changes

* test nested ppx

* document payload test

* some more tests

* resolve final comments

* add test on floating attribute

* add test for attribute in an extension's payload

* Fix function syntax

* merlin overrides: merlin locate support for PPXs (#171)

* merlin document support for PPXs (#167)

* initial working with tests

* temp update

* temp

* major refactors, add support for .mli files

* refactoring and cleanup

* move all logic into override_document.ml; cleanup code; add logging

* add many more tests; handle edge cases

* revert std.ml changes

* improve comments

* update more comments

* suggested changes

* test nested ppx

* document payload test

* some more tests

* resolve final comments

* add test on floating attribute

* add test for attribute in an extension's payload

* Fix function syntax

* working parsing of new merlin.document structure

* fix document tests with new merlin.document structure

* replicate merlin.document tests for merlin.locate

* generalize comments, code for all overrides

* delete duplicate files from merge conflict

* add test overriding locate of a non-PPX

* add test on overriding default behavior of document

* format

* combine document and locate tests into single file

* adjust for new attribute structure

* update comment documenting expected attribute structure

* fix typo

* remove named parameter

* merlin locate outputs absolute paths using source_root

---------

Co-authored-by: Liam Stevenson <lstevenson@janestreet.com>

* merlin overrides use interval tree instead of list (#173)

* interval tree interface

* interval tree implementation and basic test

* use array for construction, add more tests

* add and update comments

* fix error messages, validate inputs

* use list instead of array, add tests, add comments

* integrate into overrides.ml

* add test for empty tree

* resolve requested changes

* add test for zero-width interval

* undo test reformat

* update outdated comment

* update another oudated comment

* update comment

* Cache overrides interval trees using Phase_cache (#176)

* implementation but failing tests

* working overrides caching with tests

* add USE_PPX_CACHE, add all cache entries to tests

* update comments

* use Option.map

* move comment

* fix overrides caching mechanism, functorize caches, add cache invalidation test

* add comments, rename

* cleanup test output, and resolve other comments

* remove unnecessary sed commands from tests

* add commentsg

---------

Co-authored-by: Max Wang <orangemax888@gmail.com>