Add LibPath for shared library initialization

Currently you need to put the shared libraries in one of a few specific places, some of which require administrator access. We should add an optional `LibPath` variable to the `Math` namespace. If specified, this is where `Math` will look first for the shared libraries.

Ideally I'd like to see the following search hierarchy implemented:

- LibPath, if non-empty
- the directory with the dyalog executable
- the current directory
- the Windows\System directory (Windows only)
- the Windows\ directory (Windows only)
- a directory in the operating system's PATH
- the directory where Math.dyalog was loaded from (if available)
- the directory of the current workspace (if applicable)