GPIO, SPI and 64-bit support, platform channel refactor

[ardera]

GPIO

• plugin is called flutter_gpiod. I haven't yet published it to the dart/flutter package repo pub.dev yet. Need to fix up some documentation and do testing on some edge cases.
• plugin source code is already uploaded here, so if you really need GPIO, you can clone it and start using it right away.
• GPIO is implemented as an interface to libgpiod. This means you can use flutter_gpiod on any linux device, not just Raspberry Pi. (provided it has linux kernel drivers for the GPIO chips)
• Downside to this is that pin numbering is a little bit harder now. The new linux GPIO chardev interface changed the old 1-level hierarchy (globally unique pin numbers) to a 2-level hierarchy (gpio chip -> gpio line offset in this chip)
• Also, the features you can use depend on your linux kernel and libgpiod version. Pullup/Pulldown resistor support was only recently added, it hasn't yet arrived on Raspbian yet.
• The dart interface is experimental right now. Breaking changes will probably happen until v1.0.
• partially implements GPIO access #9

SPI

• very basic SPI plugin, using the linux spidev interface.
• supported: setting/getting SPI mode, speed, bits per word. full-duplex transfer.
• haven't yet uploaded the source code.
• experimental
• the SPI driver for the Raspberry Pi seems to be kinda buggy. More precisely, chip-select wasn't activated by the driver when doing a transfer. A temporary workaround is to request the chip-select pin as output and put a high-voltage on it. (Don't pull the voltage low when doing a transfer, only request it)

Platform channel refactor

• all methods of the platform channel interface now start with platch_.
• added some QoL methods for responding to method calls and sending events on an event channel. Also made sending and responding to platform messages thread-safe.
• renames: struct ChannelObject --> struct platch_obj
• struct StdMsgCodecValue --> struct std_value
• struct JsonMsgCodecValue --> struct json_value
• and much more. The naming is now somewhat consistent.

64 bit support

• should work on 64bit platforms now (untested)
• implements 64bit Ubuntu 19.10 support? #33