trace: Remove default trace level and make levels mandatory on span! macro (#1025)

## Motivation 

Was determined that having the span! macro default to the TRACE level is
probably not ideal (see discussion on #952). 

Closes #1013

## Solution 

Remove default trace level and make log lvl mandatory on span! macro,
and add the respective `trace_span!`, `debug_span!`, `info_span!`,
`warn_span!` and `error_span!` macros that behave as span! macro, but
with defined log levels

## Notes 

I think this is it, also removed some captures that were repeated, and
some testcases that also seemed repeated after adding the mandatory log
level, but please review it, if more tests or examples are needed happy
to provide (tried to find a way to get the generated macros log level,
but didn't find one, if there is a way i can add tests to assert that
the generated macro has the matching log level ). thanks

tokio-trace/src/span.rs

@@ -12,9 +12,10 @@
 //! if the span exists:
 //! ```
 //! # #[macro_use] extern crate tokio_trace;
+//! # use tokio_trace::Level;
 //! # fn main() {
 //! let my_var: u64 = 5;
-//! let mut my_span = span!("my_span", my_var = &my_var);
+//! let mut my_span = span!(Level::TRACE, "my_span", my_var = &my_var);
 //!
 //! my_span.enter(|| {
 //!     // perform some work in the context of `my_span`...
@@ -78,9 +79,10 @@
 //! exists. For example:
 //! ```
 //! # #[macro_use] extern crate tokio_trace;
+//! # use tokio_trace::Level;
 //! # fn main() {
 //! {
-//!     span!("my_span").enter(|| {
+//!     span!(Level::TRACE, "my_span").enter(|| {
 //!         // perform some work in the context of `my_span`...
 //!     }); // --> Subscriber::exit(my_span)
 //!
@@ -96,10 +98,11 @@
 //! time it is exited. For example:
 //! ```
 //! # #[macro_use] extern crate tokio_trace;
+//! # use tokio_trace::Level;
 //! # fn main() {
 //! use tokio_trace::Span;
 //!
-//! let my_span = span!("my_span");
+//! let my_span = span!(Level::TRACE, "my_span");
 //! // Drop the handle to the span.
 //! drop(my_span); // --> Subscriber::drop_span(my_span)
 //! # }