reviving instancemethods

[matthias314]

The purpose of this PR is to finally add instancemethods to master. This function was created by @algunion in #50898 and #51068 in response to a question of mine on Discourse. @algunion seems to be busy at the moment and said that I could complete the PR.

Recall that the idea of instancemethods(T::Type) is to return all methods for instances of T without the need to instantiate any object. I believe I have addressed all comments made in #51068. Some remarks about the current version:

• The code for methods contains the line

julia/base/reflection.jl

Line 1187 in 9ea29d9

world == typemax(UInt) && error("code reflection cannot be used from generated functions")

which is currently not in instancemethods. Does it need to be added?

• The following gives an error at present:

julia> length(instancemethods(Function))   # OK
24726

julia> length(instancemethods(Any))   # EDIT: OUTDATED
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty
   @ ./Base.jl:49 [inlined]
 [2] instancemethods(t::Type, tmatch::Any, mod::Nothing)
   @ Base ./reflection.jl:1275
 [3] instancemethods
   @ Base ./reflection.jl:1283 [inlined]
 [4] instancemethods(t::Type)
   @ Base ./reflection.jl:1283
 [5] top-level scope
   @ REPL[9]:1

EDIT: Besides Function (which is already dealt with), also Any and Core.Builtin have the property that Base.typename(T) has no field mt. This is fixed now.

EDIT 2: instancemethods(Core.Builtin) gives an empty method list although builtin functions are instances of this type. Is this a problem?

• If/when the PR gets accepted, we need to add something to NEWS.md. Where would it go there? To "New language features", to "New library features" or elsewhere?