Allow transformers-0.6

Bump bounds to acccomodate base-4.18

Check for termios.h existence

Thanks to @fluesvamp for bringing these to my attention.

Bump base upper bound

If termios.h doesn't exist (e.g. in wasi-libc), we should be using a
stub instead, otherwise hsc2hs panics at compile-time.

As noted by GHC #20085, the ccall calling convention does not support
variadic functions.

This will avoid future breakage

See https://downloads.haskell.org/ghc/8.10.3/docs/html/users_guide/using-warnings.html?highlight=wcompat#ghc-flag--Wcompat-unqualified-imports

Win32 provides a few console primitives as of the 2.9 release, resulting
in name shadowing warnings with our own local versions. Hide those from
Win32 to avoid the warnings.

* Use gsWidth to calculate the width of completions

* Update variable names and comments to mention "width", not "length"

Allow base-4.15

See GHC Proposal 287:
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst

Also bump base upper bound.

See GHC Proposal 287:
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst

See GHC Proposal 287:
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst

GHC's build currently can't handle packages with both a library and
a binary.  Help out by allowing a build to disable the executable,
which is only needed for the test.  (So disable the test as well.)

Also bump the version to 0.8.0.1.

* Fix wcwidth to handle non-BMP characters on Windows

* Encode String into UTF-16 when writing to Windows console

* Decode UTF-16 when processing input from Windows console

There are also a couple of other monads that could plausibly have this
instance, but since none of them seem to be exported I left them alone.

Also test ghc-8.6.3 in Travis CI instead of 8.6.1.