Update mangling scheme syntax

[tmiasko]

• Remove type from constant placeholders (removed in Support signed integers and char in v0 mangling rust#77554).
• Add encoding of bool values, char values, and negative integer values (added in Permit ty::Bool in const generics for v0 mangling rust#77452, Support signed integers and char in v0 mangling rust#77554).
• Make <binder> optional in <fn-sig> and <dyn-bounds> productions (since the initial implementation).
• Use <namespace> for namespace production, instead of a mix of <ns> and <namespace>.
• Use = to define <fn-sig> rule.

Fixes #3112.

[michaelwoerister]

Thanks, @tmiasko!

@eddyb, @varkor: Do you see anything in here that is still considered to be too unstable?

[varkor]

@eddyb, @varkor: Do you see anything in here that is still considered to be too unstable?

@eddyb is definitely the better person to ask – I was just following their instructions for the modifications I made! But from what I can tell, the changes look good to me.

[eddyb]

@eddyb, @varkor: Do you see anything in here that is still considered to be too unstable?

@eddyb is definitely the better person to ask – I was just following their instructions for the modifications I made! But from what I can tell, the changes look good to me.

Yupp, all good! Sorry I didn't see this earlier.