[superseded] {.ignoreNotNil.}: escape hatch for not nil

[timotheecour]

goal

escape hatch for not nil annotations, to avoid create worse problems such as contorted code or code with bad performance

example

{.push ignoreNotNil.}
... # assume the programmer knows what he's doing
{.pop.}

concrete example

see https://github.com/nim-lang/Nim/pull/13808/files/4f6cdd797cc0db0410644e7f1216e5264968deeb#diff-f48932f809aa3e1ed2576a4fdd754e26
where {.ignoreNotNil.} would avoid not nil causing a performance regression such as nim-lang#13448

[timotheecour]

superseded by nim-lang/RFCs#317