=default type bound operator to override default and var initialization

[timotheecour]

#252 was already accepted in the form of #252 (comment)

This leaves open another point, which is initialization for non-object types (as was suggested in #252 (comment))

proposal

allow type bound =default operator to override default and var initialization.
it must be a compileTime proc (or at least compiler will only call it at CT).

invariant: var a: T is always equivalent to var a = T.default

example 1

# sub.nim:
type Bar* = distinct bool
proc `=default`(t: typedesc[Bar]): Bar {.compileTime.} = true.Bar
  # type bound operator, so doesn't need to be exported

# main.nim
import sub
var a1 = Bar.default
doAssert a1.bool
var a2: Bar
doAssert a2.bool

example 2

in system.nim, add

proc `=default`(t: typedesc[range]): t = t.low

so that:

var a: range[10..20]
static: doAssert a == 10

[cooldome]

Why not =init(x: var T) as proposed in #252?. It is more efficient, no extra move/copy operation required

[timotheecour]

why would it be more efficient? =default is {.compileTime.} and affect semantics of var initializaiton, so that:

var a: Bar
  # a is not zero-initialized, it's directly initialized to `=default(Bar)`,
  # possibly from ROM

[github-actions]

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