etl::span: Add advance(), copy(), reinterpret_as()

[rolandreichweinbmw]

This commit adds to etl::span:

• advance()
• copy()
• reinterpret_as()

including tests.

[jwellbelove]

Is the purpose of reinterpret_as to map, for example, a buffer of char to a range of int, and the other way round?
Shouldn't there to be a check for size multiple and possibly alignment, so that, say, seven int8_t at an odd alignment cannot be reinterpreted as a unit32_t?

i.e.

etl::array<int8_t, 7> buffer;
etl::span<int8_t) sp1(buffer.begin(), buffer.end());
auto sp2 = sp1.reinterpret_as<unit32_t>();

uint32_t i = *sp2.begin(); // Is this aligned correctly!

[rolandreichweinbmw]

Right. I will add this.