Type Inference & Performance Improvements

[uniment]

Improve performance:

1. Restore type information to MapCallback and ObserverFunction
2. Allow method specializations for functions which are likely to be called many times
3. Eliminate unnecessary invokelatest

With Julia 1.9's native code caching, some compile-time optimizations that had previously been helpful have become counterproductive.

Before this PR:

julia> using BenchmarkTools
       using Observables: Observable, @map
       const a = Observable(1.0);  @btime notify(a)
       const b = @map &a+1;        @btime notify(a)
       const c = @map &a+2;        @btime notify(a)
       a[], b[], c[]
  5.100 ns (0 allocations: 0 bytes)
  575.275 ns (10 allocations: 160 bytes)
  1.210 μs (20 allocations: 320 bytes)
(1.0, 2.0, 3.0)

After this PR:

julia> using BenchmarkTools
       using Observables: Observable, @map
       const a = Observable(1.0);  @btime notify(a)
       const b = @map &a+1;        @btime notify(a)
       const c = @map &a+2;        @btime notify(a)
       a[], b[], c[]
  5.200 ns (0 allocations: 0 bytes)
  30.723 ns (1 allocation: 16 bytes)
  55.193 ns (2 allocations: 32 bytes)
(1.0, 2.0, 3.0)

Because invokelatest is still used in notify, it is not necessary to use it in the functor for MapCallback. Thus this PR does not affect issue #50, which remains closed.