MicroPython: Avoid heap allocations in all C++ modules.

[Gadgetoid]

As of micropython/micropython@c80e7c1 MicroPython's linker script includes a "greedy" heap, which attempts to use all free system RAM for MicroPython's heap.

Whereas previously we carefully avoided large heap allocations due to the very limited amount of heap available to C++ extensions, we must now avoid all allocations.

This changeset cleans up some uses of std::string which were implicitly allocating strings on heap. It does this by switching to string_view, wrapping the existing byte array and length returned by GET_STR_DATA_LEN instead of copying them into a std::string.

Additionally set_font() performs Hershey font lookups via a new if/else table, since the std::map<std::string, const *font_t> lookup table could not be used without constructing a string and risking a heap allocation.

Finally I have removed the completely ridiculous and unnecessary conversion to/from a std::string in our jpegdec binding's open_file functionality.

To make string_view available, all MicroPython builds needed to be forced to use C++17, this is achieved in their respective CMake files.

In all cases I believe std::string will implicitly cast to std::string_view so our examples will not break, and C++ code has always been set to C++17.

[Gadgetoid]

This is a pre-requisite to bumping all of our MicroPython builds to the latest edge commit and - indeed - keeping ahead of MicroPython changes so we can incorporate Bluetooth when it arrives.

Once merged, relevant commits from https://github.com/pimoroni/badger2040/tree/test/micropython-edge can be moved to pimoroni/badger2040#2 (and the GitHub actions .yml targeted back at the main branch here)

[Gadgetoid]

Pico RGB Keypad, Pico Scroll and Pico Unicorn have all been converted to MicroPython class-style modules. init() changes to a new class instance. For example this:

import picounicorn

picounicorn.init()

Would become:

from picounicorn import PicoUnicorn

picounicorn = PicoUnicorn()

[Gadgetoid]

Looks like interstate75.py is not included in Pico W builds and should be. The MicroPython module .cmake files need tidying again since they've got quite out of hand.