Relax type of callback arguments to Router methods

[compiler-errors]

This PR relaxes the type of the callback

func: fn(Request, RouteContext<D>) -> T

into

func: impl Fn(Request, RouteContext<D>) -> T + 'static

which is an argument-position impl trait type (APIT), since I don't see any reason for requiring the passed-in callback is a function pointer, since we're immediately boxing it up and erasing the type.

Specifically, this ensures that you can pass a (currently nightly-only) async closure (async ||), which currently does not support being coerced to a fn ptr type. I'd like to avoid needing to implement that functionality that if possible.

[compiler-errors]

Alternatively, this doesn't need to be an APIT, but could also just be a generic parameter.

Suggested change

	pub fn head_async<T>(
	mut self,
	pattern: &str,
	func: impl Fn(Request, RouteContext<D>) -> T + 'static,
	) -> Self
	where
	pub fn head_async<T>(
	mut self,
	pattern: &str,
	func: F,
	) -> Self
	where
	F: Fn(Request, RouteContext<D>) -> T + 'static,

[kflansburg]

I guess my only concern is if the 'static bound will break existing code in any circumstance.

[compiler-errors]

It shouldn't matter for closures or fn item types but would matter for fn pointer types, but also I missed that we really only need to restrict this to + 'a.