Add arbitrary-precision integer to std

A few notes on the implementation:

 - Any unsigned power of two integer type less than 64 bits in size is supported
 as a Limb type.
 - The algorithms used are kept simple for the moment. More complicated
 algorithms are generally only more useful as integer sizes increase a
 lot and I don't expect our current usage to be used for this purpose
 just yet.
 - All branches (practically) have been covered by tests.

See https://github.com/tiehuis/zig-bn/tree/986a2b3243d0454b8430a6adf4ad48611850c1b8/bench
for rough performance comparison numbers.

Closes #364.

CMakeLists.txt

@@ -464,6 +464,7 @@ set(ZIG_STD_FILES
     "math/atan.zig"
     "math/atan2.zig"
     "math/atanh.zig"
+    "math/big/int.zig"
     "math/cbrt.zig"
     "math/ceil.zig"
     "math/complex/abs.zig"

std/math/big/index.zig

@@ -0,0 +1,5 @@
+pub use @import("int.zig");
+
+test "math.big" {
+    _ = @import("int.zig");
+}