This changes the event macro (and all the upstream macros like info!),
so that it only uses each field once when the log feature is enabled.
Previously, the `field` token trees would be substituted into the log
macro invocation and the `ValueSet`, potentially meaning they are
executed more than once. This change generates the log using the
`ValueSet` meaning the token trees are only substituted once.

Fixes: tokio-rs#196, tokio-rs#1739

Fixes the span macro to only substitute the field expressions once when
the log feature is enabled. This stops enabling the log feature causing
"use of moved value" errors and other odd behaviour.
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This reverts commit bc81ef5. The
issue with the event macro does not apply to the span macro.