write system.abs in nim, not magic

[timotheecour]

refs #476 (comment)

subtlety

before PR, nim r main does't raise
after PR, nim r main raises OverflowDefect because abs is now a regular proc.
note, if i change abs to a template, this would preserve pre-existing behavior, ie not raise here.

when true:
  {.push overflowChecks: off.}
  proc main()=
    let a = abs(int64.low)
  main()
  {.pop.}

template abs*[T: SomeSignedInt](x: T): T =
  let x2 = x
  if x2 < 0: -x2 else: x2

vs:

func abs*[T: SomeSignedInt](x: T): T {.inline.} =
  if x < 0: -x else: x

note

if we want to make an inner proc (eg abs) be affected by a pragma (eg overflowChecks:off), there are 2 ways:

• use a template (simplest but in the general case might not be desirable in all cases)
• use a proc (generic or not), but make it implicitly generic on the "environment", in this case the pragma overflowChecks:off, so that 2 distinct procs could end up being instantiated in the same program: one with overflowChecks:on and one with overflowChecks:off

note 2

global flags -d:danger aren't affected by this, this is just regarding semantics for a local {.push overflowChecks: off.}

note 3

maybe implicitly generic idea (depending on "environment") could be used in other contexts, eg to generate 2 distinct procs depending on whether we're in vm or not (eg for improving nimvm semantics)

[ringabout]

@timotheecour
what's the state of this PR?

We have some other stuffs which should be implemented as magics such as isTopLevel, getProcname.