generics (in the compiler written in Finch)

fn max[T](T a, T b) -> T      struct Stack[T] { []T items }      fn Stack[T].push(T x)

The type is worked out from the arguments (max(3, 4), first([1, 2])) and written out
where it cannot be: max[float](1, 2.5), Stack[int](), Pair[str, int](...). Each use makes
its own copy, so a generic costs nothing at run time. Several parameters
(Pair[A, B]), nesting (Stack[Pair[int, str]]), methods, and printing (Stack[int](items: ...))
work. An error inside a generic says which version it was building.

boot.sh: 37 passed, 0 failed (stage 2 and 3 identical).

tests/new/generics.out

@@ -0,0 +1,10 @@
+9 2.5 pear b
+2.5
+x 7 3 1
+<5> <hi> <Point(x: 1, y: 2)> <[1, 2]>
+["ab", "ab", "ab"] [1.5, 1.5] [7, 7]
+20 1 Stack[int](items: [10])
+Stack[str](items: ["ann", "bob"]) 2
+1
+Pair[str, int](first: "age", second: 31) Pair[int, str](first: 31, second: "age") 32
+Stack[Pair[int, str]](items: [Pair[int, str](first: 1, second: "one")])