This silences binding errors due to keys not found in the current
termcap config in the default fish bindings.
Closes#4188, #4431, and obviates the original fix for #1155
It was necessary to re-implement builtin_bind as a class in order to
avoid passing around the options array from function to function and
as adding an opts parameter to `get_terminfo_sequence` would require
otps to be passed to all other builtin_bind_ functions so they could, in
turn, pass it to `get_terminfo_sequence`.
Running the tests on travis revealed that some compilers (or at least
with some options) call the wrong struct constructor if there is more
than one struct with the same name but differing definitions.
