Problem:    Calling a lambda is slower than evaluating a string.
Solution:   Make calling a lambda faster. (Ken Takata, closes vim/vim#5727)
https://github.com/vim/vim/commit/f10806b25090879fdc1a86cc0da2f4f34fd21921

Port "uf_flags" constants from patch 8.2.1054 to sync with Vim.
Port user_func_error() from patch 8.2.0149.
Port Test_lambda_scope() changes from patch 8.1.0736 so that it passes.

Problem:    Testing lambda with timer is slow.
Solution:   Do not test timer accuracy, only that it works. (Daniel Hahler,
            closes vim/vim#4723)
https://github.com/vim/vim/commit/9bc4dde45d45df732953491d0f2c3fd3b10a627e

Problem:    Error when defining a Lambda with index of a function result.
Solution:   When not evaluating an expression and skipping a function call,
            set the return value to VAR_UNKNOWN.
https://github.com/vim/vim/commit/b4518563c73460150344a57879bf5b22cb8b1c77

    From test_alot.vim:
    Found errors in Test_lambda_with_timer():
    First run:
    function RunTheTest[35]..Test_lambda_with_timer line 19: Expected True but got 0
    Second run:
    function RunTheTest[35]..Test_lambda_with_timer line 19: Expected True but got 0

previously: #9220

- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b02
  ref de13113dc16e
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.
- Call test_garbagecollect_now(), as Vim does.

- window_split_tab_spec.lua: Put cursor at bottom of :terminal buffer so
  that it follows output.
- inccommand_spec.lua: Increase timeout to allow 2nd retry.
- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b02
  ref de13113dc16e
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.

Problem:    Using function() with a name will find another function when it is
            redefined.
Solution:   Add funcref().  Refer to lambda using a partial.  Fix several
            reference counting issues.

https://github.com/vim/vim/commit/437bafe4c8a83ed71ee006eda7f54b65a90f0d4c

Problem:    Closure function fails.
Solution:   Don't reset uf_scoped when it points to another funccal.

https://github.com/vim/vim/commit/580164481924ed8611eb79f0247a0eb1ca0b3b9a

Problem:    User defined functions can't be a closure.
Solution:   Add the "closure" argument. Allow using :unlet on a bound
            variable. (Yasuhiro Matsumoto, Ken Takata)

https://github.com/vim/vim/commit/10ce39a0d52272a3dfff2feb8c631529f29e6740