trace: Add static level filtering (#987)

## Motivation

`tokio-trace` should have static verbosity level filtering, like the
`log` crate. The static max verbosity level should be controlled at
compile time with a set of features. It should be possible to set a
separate max level for release and debug mode builds.

## Solution

We can do this fairly similarly to how the `log` crate does it:
`tokio-trace` should export a constant whose value is set based on the
static max level feature flags. Then, we add an if statement to the
`span!` and `event!` macros which tests if that event or span's level
is enabled.

Closes #959

tokio-trace/test_static_max_level_features/Cargo.toml

@@ -0,0 +1,10 @@
+[workspace]
+
+[package]
+name = "test_cargo_max_level_features"
+version = "0.1.0"
+publish = false
+
+[dependencies.tokio-trace]
+path = ".."
+features = ["max_level_debug", "release_max_level_info"]