value/test_worker: Test remote function execution
Fix array concatenate to work with const and add some utility functions to Nested

* Change concatenate to work on Array<const T>
* Add Nested::front and Nested::back methods
* Add Nested::append_to_back and Nested::extend_back for unfrozen nested arrays
* Add some very simple unit tests for Nested arrays
Merge branch 'master' of github.com:otherlab/core into exact
Move predicate helper structs outside of function scope

Clang currently allows member templates in local classes, but this is technically in violation of the standard and isn't portable to gcc.