Get flowmacro compiling using stable

[andrewdavidmackenzie]

Without it we can't install using release buil dusing homebrew or other builds?

cargo +nightly build --release -p flowstdlib
   Compiling flowmacro v0.92.0 (/Users/andrew/workspace/flow/flowmacro)
error[E0554]: `#![feature]` may not be used on the stable release channel
 --> flowmacro/src/lib.rs:2:12
  |
2 | #![feature(proc_macro_span)]
  |            ^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0554`.
error: could not compile `flowmacro` due to previous error

[andrewdavidmackenzie]

Blocked on stabalizing the span methods needed