feat: compress indices

[KnorpelSenf]

Object and array indices often have a sequence of numbers (

oson/oson.test.ts

Line 93 in ab4bb0e

["", 1, 2, 3, 4, 5, 6],

) or a single number repeated many times (

oson/oson.test.ts

Line 58 in ab4bb0e

assertEquals(listify([1, , , , -1, ,]), [[1, -2, -2, -2, 2, -2], 1, -1]);

).

It is trivial to compress these things down into a much more space-efficient version. This will greatly reduce the number of bytes used in the serialised form. It will in no case increase this number of bytes.

The runtime complexity will remain identical. Perhaps there is a change in the constants, but it isn't clear upfront in which direction the change will happen. This should be benchmarked.