Add docstrings to internally defined types

Add docstrings to the type constructors and constructors for (=), (,),
_|_, and (). Some of these are not easy to look up from :doc, but they
show up in tooltips in certain IDEs, and :doc should be improved to make
them easier to find.
Add -e EXPR to evaluate expressions w/out REPL

The command line option follows the convention from Perl. This causes
Idris to evaluate the provided expressions, pretty-printing them without
line breaks. Then, test results need not depend on terminal widths any
longer. If multiple -e options are provided, then the expressions are
evaluated in order.
Improve the portability of reg041
Enable word-wrapping in docstrings

Now, docstrings will be rendered to the console with word-wrapping.
Docstring improvement for prop. eq.
Add note about additional docs to CHANGELOG
Merge pull request #1094 from david-christiansen/internal-docstrings

Docstrings for internal types + test portability fix