Allow passing useRecords function

[somebee]

See issue #112. This PR allows you to set the useRecords option to a function. If useRecords is a function, Packr will call this function for each object, and only use records when the function returns true.

When you have many structured objects combined with many smaller unstructured ones it can make packing smaller and faster, and unpacking substantially faster. For our real-world data we see around 10% size reductions and ~40% faster unpacking!

useRecords:(v=>!!v.id), variableMapSize: true
size: 923kb - pack 8.88069ms - unpack 6.82440ms

useRecords:(v=>!!v.id)
size: 941kb - pack 8.49143ms - unpack 5.41675ms

useRecords: true
size: 1050kb - pack 9.65734ms - unpack 10.21317ms

useRecords: false
size: 2191kb - pack 10.37920ms - unpack 16.75035ms

The only caveat I can think if is that you'll likely want to set mapsAsObjects: true if you simply replace useRecords: true with a function. If useRecords is true, all plain objects will be unpacked as plain objects since they are packed as records. But when you opt out of record packing for certain objects you need to remember that they will then be unpacked as maps now unless you set mapsAsObjects to true.

[kriszyp]

Impressive results, this looks good. Thank you for the PR!