Start of core:math/bigint

We have:
- `init` to create a new `Int`
- `init(from_integer)` to create a new `Int` and set it to `from_integer`.
- `set(Int, from_integer)` to set an `Int` to `from_integer`
- `add(dest, a, b)` to add `a` and `b` into `dest`.
- `sub(dest, a, b)` to subtract `b` from `a` and put the result in `dest`.

And a few helper functions, like:
- `is_zero`, `is_negative`, ...
- `grow`, `shrink`, `clear`, `zero`

core/math/bigint/example.odin

@@ -0,0 +1,50 @@
+//+ignore
+package bigint
+
+/*
+	Copyright 2021 Jeroen van Rijn <nom@duclavier.com>.
+	Made available under Odin's BSD-2 license.
+
+	A BigInt implementation in Odin.
+	For the theoretical underpinnings, see Knuth's The Art of Computer Programming, Volume 2, section 4.3.
+	The code started out as an idiomatic source port of libTomMath, which is in the public domain, with thanks.
+*/
+
+import "core:fmt"
+import "core:mem"
+
+demo :: proc() {
+	a, b, c: ^Int;
+	err:  Error;
+
+	a, err = init(-21);
+	defer destroy(a);
+	fmt.printf("a: %v, err: %v\n\n", a, err);
+
+	b, err = init(42);
+	defer destroy(b);
+
+	fmt.printf("b: %v, err: %v\n\n", b, err);
+
+	c, err = init();
+	defer destroy(c);
+
+	err = add(c, a, b);
+	fmt.printf("c: %v, err: %v\n\n", c, err);
+}
+
+main :: proc() {
+	ta := mem.Tracking_Allocator{};
+	mem.tracking_allocator_init(&ta, context.allocator);
+	context.allocator = mem.tracking_allocator(&ta);
+
+	fmt.printf("_DIGIT_BITS: %v\n_MIN_DIGIT_COUNT: %v\n_MAX_DIGIT_COUNT: %v\n_DEFAULT_DIGIT_COUNT: %v\n\n", _DIGIT_BITS, _MIN_DIGIT_COUNT, _MAX_DIGIT_COUNT, _DEFAULT_DIGIT_COUNT);
+
+	demo();
+
+	if len(ta.allocation_map) > 0 {
+		for _, v in ta.allocation_map {
+			fmt.printf("Leaked %v bytes @ %v\n", v.size, v.location);
+		}
+	}
+}