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 986a2b3243/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");
+}

std/math/index.zig

@@ -132,6 +132,8 @@ pub const tan = @import("tan.zig").tan;
 pub const complex = @import("complex/index.zig");
 pub const Complex = complex.Complex;
 
+pub const big = @import("big/index.zig");
+
 test "math" {
     _ = @import("nan.zig");
     _ = @import("isnan.zig");
@@ -177,6 +179,8 @@ test "math" {
     _ = @import("tan.zig");
 
     _ = @import("complex/index.zig");
+
+    _ = @import("big/index.zig");
 }
 
 pub fn min(x: var, y: var) @typeOf(x + y) {