trace: Switch to using local_inner_macros for instrumentation API (#…

…969)

## Motivation

Currently, it isn't possible to import individual macros from
`tokio-trace` using the macros 1.2 syntax:

```rust
use tokio_trace::{debug, info, span};
```

This is because these macros require that `callsite` and `enabled` are
imported as well.

## Solution

This branch resolves the problem by adding the [`local_inner_macros`]
attribute to the instrumentation API's macros. This allows other macros
from within the crate to be used without requiring them to be explicitly
imported. 

However, this also requires duplicating any macros from other sources
(such as std and `tokio-trace-core`) with wrappers due to the behaviour
of `local_inner_macros`. I've added these wrapper macros as well.

Since the macros got even longer as a result of this, I've moved them
to a separate file to make `lib.rs` easier to read. I've also wrapped
some very long lines in the macros, and removed the explicit drop of
the result of evaluating some event macros (it's no longer necessary
as all event macros now evaluate to `()`).

[`local_inner_macros`]: https://doc.rust-lang.org/nightly/edition-guide/rust-2018/macros/macro-changes.html#local-helper-macros

Fixes #968

Signed-off-by: Eliza Weisman <[email protected]>