[TODO] [WIP] new outplace operator without intermediate copies/temporaries

[timotheecour]

Not ready for review unless you want to help (eg concrete simpler implementation suggestion etc), but this seems to work well

Will eventually merge this into #13309, just a preview.
Implementation is more complex, which is the price to pay for generating efficient code: this avoids temporaries where possible; namely: consecutive elements in a .> chain generate a single temporary instead of 1 per element of the chain.

example:

@[1].>fun(2).>fun(3).process.process.>fun(4).>fun(5)

generates:

block:
  var tmp_475204 = @[1]
  fun(tmp_475204, 2)
  fun(tmp_475204, 3)
  var tmp_475205 = tmp_475204.process.process
  fun(tmp_475205, 4)
  fun(tmp_475205, 5)
  tmp_475205

more examples: see unittests

still open questions

• we could in fact avoid any copies at all in some cases (eg when leftmost expression in a chain is a nnkIdent), but that currently breaks for examples involving openArray, see D20200205T024841 in tests

[timotheecour]

Out of curiosity, does it still need to be called "outplaces" instead of just "outplace"?

replied here: #13309 (comment) ; no need to duplicate comments ;-)

[Araq]

See nim-lang/RFCs#192 for a discussion of the design.

[Araq]

RFC is shipping.