Optimized Map/Set lookups in Structure

[joerg1985]

Reduced the number of Set lookups in Structure.read / Structure.write.
Introduced a new class to replace two entries of a HashMap with one.
This reduces the number of lookups and the initial capacity has been
reduced too.

There is one additional change in line ~363 to avoid a Memory.share()
call if the size of the structure is known.

I could create a benchmark if required.

[joerg1985]

Here are some benchmark numbers, i will close and reopen this PR to rerun Travis CI.

Branch	Benchmark	Mode	Cnt	Score	Error	Units
master	write_read	thrpt	25	268977.759	± 5716.129	ops/s
structure-less-lookups	write_read	thrpt	25	307419.387	± 7757.885	ops/s

public class StructureBenchmark {
    @Structure.FieldOrder({"b0", "b1", "b2", "l0", "l1", "l2", "s0", "s1", "s2"})
    public static class MyStructure extends Structure {

        public byte b0;
        public byte b1;
        public byte b2;

        public long l0;
        public long l1;
        public long l2;

        public String s0;
        public String s1;
        public String s2;
    }
    
    @Benchmark
    public void write_read(Blackhole blackhole, MyState state) {
        MyStructure structure = state.structure;

        structure.s0 = structure.s1 = structure.s2 = "test";

        structure.write();
        structure.read();
        
        structure.s0 = structure.s1 = structure.s2 = null;
        
        structure.write();
        structure.read();

        blackhole.consume(structure);
    }
    
    @State(Scope.Thread)
    public static class MyState {
        MyStructure structure = new MyStructure();
    }
}

[dbwiddis]

This looks generally good. I've left one question inline. No need for you to close/reopen to trigger the travis builds, we can also trigger them manually when needed (if it's a single, transient failure not associated with the PR)

[joerg1985]

Okay, i updated the PR and left some comments to better understand what the code does.
The benchmark numbers are still good / better than the master.

Branch	Benchmark	Mode	Cnt	Score	Error	Units
structure-less-lookups	write_read	thrpt	25	302414.960	± 23849.565	ops/s

[matthiasblaesing]

Looks good to me. Thanks for investigating and cleaning up. I'll give it a few more day to settle - and if nothing further creeps up or someone beats me to it, I'll merge sometime next week.

[matthiasblaesing]

Ah - one further request: Could you add an entry for the CHANGES.md file?

[joerg1985]

The latest push added the entry to CHANGES.md

[matthiasblaesing]

Thank you - pulling this in.