Consider adding `ElementType` to replace IonJava's `IonType`

[dlurton]

Problems with IonType:

• It includes DATAGRAM which IonElement does not support
• It has static methods like IonType.isText(type) which is jarring and awkward to use from Kotlin.

It might be better to have ElementType, something like:

enum class ElementType(
    val isText: Boolean, 
    val isSequence: Boolean, 
    val isBinary: Boolean
) {
    NULL(false false false),
    BOOL(false, false false),
    INT(false, false false)
    FLOAT(false, false false)
    DECIMAL(false, false false)
    TIMESTAMP(false, false false)
    SYMBOL(true false false)
    STRING(true, false false),
    CLOB(false, false, true),
    BLOB(false, false, true),
    LIST(false, true, false,
    SEXP(false, true, false),
    STRUCT(false, true, false);

    /** Converts this [ElementType] to [IonType]. */
    fun toIonType(type: IonType) = when(this) { ... } 
}

/** Converts this [IonType] to [ElementType]. */
fun IonType.toElementType() = when(this) { ... } 

Then instead of:

if(IonType.isText(element.type)) { ... }

We can:

if(element.type.isText) { ... }