monero.dart clean implementation

[MrCyjaneK]

depends on https://github.com/cypherstack/flutter_libmonero/pull/64

build instructions:

$ git clone https://github.com/MrCyjaneK/stack_wallet --branch cyjan-monerodartv2 --recursive
$ cd stack_wallet
$ rm -rf crypto_plugins/flutter_libmonero
$ git clone https://github.com/mrcyjanex/flutter_libmonero --branch cyjan-monerodartnew crypto_plugins/flutter_libmonero
$ cd scripts && ./prebuild.sh && cd ..
$ cd scripts/linux && ./build_all.sh && cd ../..
$ cd scripts/android && ./build_all.sh && cd ../..