Follows discussion at #2389

This decoder works, so I thought I'd start the PR with what I've got. Aside from whatever else comes up, there are two issues with it I'd appreciate direction for:

• Are there better utility functions for converting the raw bits to symbols? I didn't have much luck finding them
• My use of a 'type' json field doesn't fit with fields described in DATA_FORMAT.md, should I be using 'subtype' instead?

Remove the "type" key you have. There is no need for that info, it's either packets with "heat" or empty packets.

I can rewrite the bit decoder later, basically I would use the fact that when aligned a byte will hold exactly 2 bits, i.e. it can only ever be 55, 5a, a5, aa. Otherwise using extract_bits_symbols() will also work https://github.com/merbanan/rtl_433/blob/master/include/util.h#L84

I've updated the code with the corrections. The decoder matches against byte values now, but I'm happy for the it to be rewritten :)

Looks good. In the long term we might want to put the bits into a uint8_t[] instead of the (large) bitbuffer_t. But that's an optimization for later (e.g. manchester_decode() also needs that refactor some day).