Attempt to replace `pika::invoke` and `pika::function` with `std` equivalents

[msimberg]

Assuming we don't require our own version for e.g. CUDA and the replacements actually work, we need to take care that the following don't get worse:

• compile time
• debug bloat
• runtime performance

[msimberg]

This can be done as part of cleaning up the functional module in #16.

[msimberg]

The main blocker here is supporting annotations through the standard library facilities.