Minor updates

* Rename getsyms->getvars, varsym->getvar
* Do not include expression head in getvars results
* Export subslicedim so that _eval can find it when evaluating constants
Unexport subslicedim again, and fully qualify name in code.
Improve code generation for testing constant values.
Functionality updates/fixes

* Use subarrays in all cases of matching arrays,
  except when when selecting a single index
* Generate default result when not using =>
  * (var1, var2, ...) when the match expression contains
    capture variables
  * true otherwise
* Also allow captured variables to "escape" when not using =>
Merge branch 'updates'
Continued updates:

* Change matrix tests to not use :(=>)
* If only one match variable, return that variable not wrapped in a tuple
Update README.md to match recent changes.