Immutable struct instances factory function

This allows for the direct creation of frozen / immutable struct
instances. A helper factory function is inserted in the declaring
module.

The helper function name is 'Immutable'<name of the struct>.
Simplifications and generate info for parameters
Code cleanups
Correct local variable table generation

This has been a long standing issue. Somehow I got it all wrong: you
just have to visit start / end labels, and then visit the local
references. With this code it gets really straightforward.
Merge pull request #103 from jponge/wip/fix-debug-vars

Variables debug symbol fixes
Merge pull request #102 from jponge/wip/immutable-struct-factory

Immutable struct instances factory function