unmarshal: optimize slice capacity growth in unmarshalArray function

[semihbkgr]

fixes #86

[DHowett]

I love it. Thank you :)

[DHowett]

I should note before merging, though, that this doesn't meet the criteria you laid out in #86 - when the cap is 5, it still grows to 10 rather than 8.

Is that still acceptable?

[semihbkgr]

If the slice has already been allocated with an underlying array (e.g., the slice capacity is 5), and the parsed elements cannot fit into the current capacity (e.g., 7 elements), then the capacity should grow by doubling (resulting in a new capacity of 10).

However, if the slice has not been allocated yet (e.g., it is nil, so the slice capacity is 0), then the slice should grow logarithmically. For example, when the array has 7 elements, the slice capacity should be 8.

This is actually what I planned with these changes.

[DHowett]

Ah, perfect

[semihbkgr]

This is also the same as how encoding/json behaves when unmarshaling slices