Add support for generic fields

[mratsim]

This fails with "Unexpected bracket expression array[" as genny tries to call getTypeInst on a A[B].

https://github.com/treeform/genny/blob/33f0e5a3e6d370c63f11ac0572f6396af684a314/src/genny/internal.nim#L92-L97

import genny

type
  Foo = object
    val: array[3, uint64]

proc sum(r: var Foo, a, b: Foo) =
  for i in 0 ..< r.val.len:
    r.val[i] = a.val[i] + b.val[i]

exportObject Foo:
  procs:
    sum(Foo, Foo, Foo)

writeFiles("bindings/generated", "test")
include generated/internal