This commit implements the 'e' half-float format: 10-bit mantissa, 5-bit
exponent. It uses native _Float16 if supported by the compiler, otherwise
uses custom bitshifting encoding/decoding routines.
Signed-off-by: Matthias Urlichs <matthias@urlichs.de>
Signed-off-by: Damien George <damien@micropython.org>