double-precision float

[prabhu-yu]

Many applications demand higher precision.
Single precision float, which is default, can not provide it.
So, in such case, we need to enable double precision.
Also, double precision enables the code running on
RPI pico to get same result as that on Desktop Python
or Desktop microPython port.

Steps to compile

# clone this repo
cd micropython
git submodule update --init -- lib/pico-sdk lib/tinyusb
make -C mpy-cross
cd ports/rp2
make submodules
make
find . -name "*.uf2" # this is your file. typically ./build-PICO/firmware.uf2. Flash this to pico board