Let inferGenericTypes continue if a param is already bound

[SirOlaf]

The current inference does not support code like below as T or E will be bound already which causes an early return. Originally this PR was gonna try to convert the types for string to cstring conversion, but that causes more problems than it solves, so now it simply lets the inference continue instead of returning.

{.experimental: "inferGenericTypes".}

type
  ResultKind = enum
    Ok, Error

  Result[T, E] = object
    case kind: ResultKind
    of Ok:
      okVal: T
    of Error:
      errVal: E

proc ok[T, E](x: T): Result[T, E] =
  Result[T, E](kind : Ok, okVal : x)

proc err[T, E](x: E): Result[T, E] =
  Result[T, E](kind : Error, errVal : x)

proc doStuff(): Result[int, string] = 
  err("Error")

[SirOlaf]

The conversion part might actually be impossible because of an ambiguous evaluation order for a case like in timezones package

var x: int64
x = read(int32) # call params say int32 but return type wants int64

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 67122a9

Hint: mm: orc; opt: speed; options: -d:release
168497 lines; 8.269s; 611.668MiB peakmem