Investigate usage of java collections instead of ImmutableList

[mvanaken]

In metal we use ImmutableList and ParseGraph as data structures. This has the following drawbacks:

• difficult to understand when processing the parse result
• difficult to use when creating custom expressions
• ordering problems when processing an ImmutableList (switches order)
• performance issues

It was implemented to keep metal as immutable as possible to avoid threading issues. Performance was not a prioritized concern.

The following ideas to improve the problems:

• Use capsule, which has implemented immutable data structures using java collection interfaces: https://github.com/usethesource/capsule/ (a List is a Map with integer keys?)
• Implement streaming-ish methods to ImmutableList and improve code that iterate over the immutable list
• Replace direct usage of head and tail with ... ? Goal: phase out the usage of our custom ImmutableList

TODO:

• Try this collections brute force: this requires some days to really discover what the challenges are and best approach is.