Idea to reduce binary size: use std::string_view

[Staars]

When reviewing my own code I stumbled upon the usage of std::string, which is a relatively expensive operation in a time critical section of my driver.
We could prevent allocation here by using std:string_view that passes the reference to the underlying data, which IMHO is a more efficient method for ISR-like callbacks. Basically I never really used std::string in my later code and converted back to const char * anyway.

I already tested the following variant of non-breaking API addition to NimBLEAdvertisedDevice:

#include <string_view>

std::string_view NimBLEAdvertisedDevice::getPayloadViewByType(uint16_t type, uint8_t index) const {
    size_t data_loc;
    if (findAdvField(type, index, &data_loc) > 0) {
        const ble_hs_adv_field* field =
            reinterpret_cast<const ble_hs_adv_field*>(&m_payload[data_loc]);
        if (field->length > 1) {
            return std::string_view(
                reinterpret_cast<const char*>(field->value),
                field->length - 1
            );
        }
    }
    return {}; // empty view
}

std::string_view NimBLEAdvertisedDevice::getNameView() const {
    return getPayloadViewByType(BLE_HS_ADV_TYPE_COMP_NAME, 0);
}

(header change accordingly)

This would already carve 28 bytes out of my final binary with the small needed changes on the application side and should run faster, although I did no measurements yet.

That way it would be a non-breaking change and maybe you would consider to use this pattern in other parts of your library too.

[h2zero]

Thanks @Staars, I agree, there are some optimizations to make using newer C++ versions. For now, since this is still used with older Arduino cores, I have delayed making these changes. I will look to do this in the future for sure though.