Establish stabilization path for macro_rules feature

[brson]

macro_rules needs to be stabilized before general plugins. At the least we can just put these under a different feature.

[pnkfelix]

As the description says, it is very likely we are going to want to allow use of macro_rules! in the stable release channel. (See #16678 )

But there are a number of details that remain to be resolved. For example:

• Will macro export also be made "stable" ?

[pnkfelix]

Assigning P-backcompat-lang, 1.0.

(This issue is still quite open-ended; much remains to be resolved. but we want to ensure we track the work that needs to be done here.)

[aturon]

@kmcallister has some thoughts here.

[alexcrichton]

This is now finished!

cc #20482
cc #20563

[aturon]

I believe this is now complete. Nominating for closure.