Implement conditional support in terminfo, along with a few other related operators.
Fix implementation of non-commutative arithmetic operators.
Remove all known cases of task failure from `terminfo::parm::expand`, and change the method signature.
Fix some other miscellaneous issues.
Implement the %?, %t, %e, and %; operators. Also implement the %<, %=,
%> operators, without which conditionals aren't very useful.
Fix the order of parameters for the arithmetic operators.
Implement the missing %^ operator.
Replace all potentially-failing operations with Err returns and add
tests.
Remove the Char parameter type; characters are represented as Numbers.
Fix integer constants to work properly when there are multiple constants
in the same capability string.
Tweak loop to use iterators instead of indexing into cap.
The confusing mixture of byte index and character count meant that every
use of .substr was incorrect; replaced by slice_chars which only uses
character indices. The old behaviour of `.substr(start, n)` can be emulated
via `.slice_from(start).slice_chars(0, n)`.
OS X's terminfo uses the hex representation of the first character of
the terminal name as the directory name.
Ubuntu seems to use /lib/terminfo instead of /usr/share/terminfo, at
least on the one machine I have access to.
