Add -Wdouble-promotion and -Wfloat-conversion for most ports to ban out
implicit floating point conversions, and add extra Travis builds using
MICROPY_FLOAT_IMPL_FLOAT to uncover warnings which weren't found
previously. For the unix port -Wsign-comparison is added as well but only
there since only clang supports this but gcc doesn't.
In this port JavaScript is the underlying "machine" and MicroPython is
transmuted into JavaScript by Emscripten. MicroPython can then run under
Node.js or in the browser.
