fix #34824, improve various cases of showing method signatures

[JeffBezanson]

The code for this was duplicated several times (more than 3, depending how you count), with all sorts of problems. Here are some examples.

Before:

julia> methods(Complex)
# 7 methods for type constructor:
[1] (::Type{Complex})(x::Real) in Base at complex.jl:16
...

julia> methods(Complex{Int})
# 7 methods for type constructor:
[1] (::Type{Complex{T}})(x::AbstractIrrational) where T<:Real in Base at irrationals.jl:34
...

julia> (Complex{T} where T<:Integer)(1.2)
ERROR: MethodError: no method matching Complex{T} where T<:Integer(::Float64)
Closest candidates are:
  Complex{T} where T<:Integer(::T) where T<:Number at boot.jl:715

julia> (::Union{Nothing,Int})(::Nothing) = nothing

julia> nothing("")
ERROR: MethodError: no method matching (::Nothing)(::String)
Closest candidates are:
  Any(::Nothing) at REPL[6]:1

After:

julia> methods(Complex)
# 7 methods for type constructor:
[1] Complex(x::Real) in Base at complex.jl:16
...

julia> methods(Complex{Int})
# 7 methods for type constructor:
[1] Complex{T}(x::AbstractIrrational) where T<:Real in Base at irrationals.jl:47
...

julia> (Complex{T} where T<:Integer)(1.2)
ERROR: MethodError: no method matching (Complex{T} where T<:Integer)(::Float64)
Closest candidates are:
  (::Type{T})(::T) where T<:Number at boot.jl:715

julia> (::Union{Nothing,Int})(::Nothing) = nothing

julia> nothing("")
ERROR: MethodError: no method matching (::Nothing)(::String)
Closest candidates are:
  (::Union{Nothing, Int64})(::Nothing) at REPL[1]:1

Fixes #34824. Replaces #35311.

Discussion questions:

• I decided to print methods on unionalls as (Foo{T} where T)(x) even though that is not currently valid input syntax. With allow where syntax in constructor definitions #35861 it would be, so I think it's ok.
• The "closest candidates" printing used to show the called type in the head of each signature, instead of the type from the definition (see third example above). That was probably intentional and I can see a case for it, but I think it's confusing because it looks like it's showing definitions that don't actually exist. I think we should show the same thing that e.g. methods(t) shows.