Enforce immutability and insertion iteration order properties of ZplCommand.parameters

[jbduncan]

This PR is a follow-up to #18.

In #18, I described a way of eliminating the need to make parameters a LinkedHashMap publicly without losing insertion iteration ordering.

This PR is a bit different to how I described it, but the main change is that ZplCommand becomes an abstract class whose constructor accepts a list of ZPL parameters to enforce insertion ordering, and ZplCommand.parameters becomes an unmodifiable map of these parameters.

Advantages

1. People can't accidentally do the following:

   override val parameters: Map<CharSequence, Any?> = hashMapOf()

   ...which loses insertion ordering.

2. People can't be naughty and do this:

   val barCode = BarCode(...)
   
   (barCode.parameters as MutableMap<CharSequence, Any?>)["foo"] = "bar"

   ...since it now throws an UnsupportedOperationException. This is due to the use of java.util.Collections.unmodifiableMap.

Drawbacks

1. ZplCommand is changed in a backwards-incompatible way, but this is probably fine as we're still on version 0.x.
2. It's arguable if java.util.Collections.unmodifiableMap is necessary. On one hand, it guards against Kotlin's read-only collections being able to be casted into mutable collections. On the other hand, it's Java-specific (though we're targeting JVM/Android anyway) and it's not idiomatic in Kotlin.

Thoughts?

[jbduncan]

As alluded to in the OP, this use of unmodifiableMap is what I'm most ambivalent about, as on one hand it stops parameters being casted into a mutable map, but on the other hand it's not idiomatic in Kotlin and just .toMap() is what most people use by default.

What do you think?