Added PackedIntArray, PackedIntSlice to std

[tgschultz]

PackedIntArray(comptime Int: type, comptime int_count: usize) type: Creates a bit-packed array of int_count integers of type Int. Bits are packed using native endianess and without storing any meta data. PackedIntArray(i3, 8) will occupy exactly 3 bytes of memory.

PackedIntSlice(comptime Int: type) type: Uses a slice as a bit-packed block of integers of type Int. Bits are packed using native endianess and without storing any meta data.

.len() gets the number of items the array/slice can hold.
.get(index: usize) and .set(index: usize, int: Int) are used for accessing the individual items.
.slice(start: usize, end: usize) creates a new PackedIntSlice(Int) instance covering the specified range of items (exclusive).
.sliceCast(NewInt: type) returns a new PackedIntSlice(NewInt) covering the same bytes but using a different bit-width integer (which must fit evenly within the old range of bits).

PackedIntArrays can be initialized from unpacked arrays or literals. PackedIntSlices initialize with a []u8 and a count of items. .bytesRequired(int_count: usize) can be used to calculate the number of bytes required in the slice for the given count.

Update: there are now also PackedIntArrayEndian, PackedIntSliceEndian, and .sliceCastEndian which allow specifying the endianess of the bit packing instead of using native.

[andrewrk]

These are likely to be moved to third party packages in the std lib audit (#1629), but for now, they're very welcome, because they provide a well-encapsulated, well-tested API that helps prevent regressions in the language, as well as an experiment in solving "packed arrays" in userland. 👍