Use fully qualified names in macros

Macro implementations should not be depending on imports done by the caller.

https://github.com/AlexEne/rust_hawktracer_normal_macro/blob/b729096df0d243a203a1e81efd917b9876d4033b/src/lib.rs#L22

A user of `rust_hawktracer` running code with edition 2018 should be able to add a scoped tracepoints simply doing
```rust
rust_hawktracer::scoped_tracepoint!(_something_to_trace)
```

This is currently not possible, so the alternative seems being
* importing all `rust_hawktracer` with the risk of polluting the namespace
* importing the symbols that `scoped_tracepoint` expects (`rust_hawktracer::{add_cached_mapping, scoped_tracepoint, ScopedTracepoint}`).