lazy semcheck

[timotheecour]

(would have a huge impact on compile times)
I implemented a POC of this a while ago in a branch, but would need revival.
for a symbol declaration, don't semcheck it until it's used in an imperative context, in fact this should be the same algorithm as the one used for cgen/jsgen, for which dead-code elimination happens automatically in a top down way

proc fn1 = discard # fn1 not semchecked yet
proc fn2 = fn1() # fn1 still not semchecked because fn2 is only declared, not used
when declared(fn1): discard # still not using fn1 in imperative context

when compiles(fn2()): discard
  # this would trigger semcheck of fn2, which would trigger semcheck of fn1
fn2() # ditto

links

• Add ceilDiv and fastCeilDiv in math nim-lang/Nim#18596 (comment)