fix List[Int].join crash; support joining int lists

[kacy]

summary

list.join(sep) lowered to pith_list_join, which reads every element as a
C-string pointer. on an Int list that dereferences integers as pointers and
segfaults (found while testing the list-method-syntax PR; it predates that
change and reproduced with the free-function form too).

now an Int-element list lowers to a new pith_list_join_int runtime function
that formats each element as decimal:

[1, 2, 3].join(",")        # -> "1,2,3"
nums.map(fn(x) => x*2).join("|")

String lists are unchanged. element types that join still cannot format
(Float, Bool, structs, ...) are now a clear compile error instead of a
crash.

how

• runtime: new pith_list_join_int formats i64 elements with a separator;
  registered in runtime-abi/runtime_functions.txt
• ir emitter: when join's receiver has an Int element kind, emit
  list_join_int instead of list_join; added to the string-returning method
  set so its result type is String
• checker: join now requires String or Int elements, reporting E219
  otherwise

what was tested

• new regression tests/cases/test_list_join_int.pith (separators, empty list,
  single element, negatives, map-then-join, and a String list)
• new invalid-checker case invalid_join_non_string asserting E219 on a
  List[Float] join
• full suite: regressions (94), invalid checker (14), examples native + self
  hosted (83 each), bootstrap fixed point — all 0 failures