Alloc-free implementation

[MathiasKoch]

VecDeque elimination

As part of the strides towards an alloc-free implementation, the first first step would be to remove the VecDeque, in favor of something like heapless::LinearMap.
Fixed

Alloc-free mqttrs

An issue on the dependant mqttrs has been opened to push for alloc-free mqtt encoders and decoders. 00imvj00/mqttrs#27
Fixed

Remove String and Vec

Once the above mqttrs has been fixed, the String and Vec from alloc can be replaced with equivalent heapless versions. 00imvj00/mqttrs#29