tracers-macros-hack

Do not reference this crate directly; see the "tracers" crate for further instructions. This crate contains the actual `proc-macro=true` crate which implements the probing macros. Because the `probe!` macro is actually a procedural macro but is used in expression position, we must use the `proc_macro_hack` crate to work around the fact that Rust doesn't natively support this usage yet. That hack requires that the actual macro be implemented in one crate, then re-exported from a second crate. Thus, to ensure that callers can simply add a dependency on `tracers_macros`, we make `tracers_macros` the re-exporting crate and this one, `tracers_macros_hack`, the actual proc macro crate. Confused? Me too.