Add support for complex Map key types

[Skn0tt]

Currently, SuperJSON is unable to serialise Map { /a/g => foo } or any other key type that's compared by reference. This is because there can be serialisability conflicts:

m = new Map()
m.set(/a/g, 1)
m.set(/a/g, 1)
m // Map { /a/g => 1, /a/g => 1 }

/* How do you serlialize this? */

Some other things that need to be accounted for when extending SuperJSON to serialise reference-compared values:

• referential equality to other values and keys needs to be persisted (e.g. added to referentialEqualities)
• what about reference cycles contained in a key?

[Skn0tt]

Closed by #37