GnoVM: Expose methods of exported struct variables to the MsgCall API

[leohhhn]

Description

If a realm has an exported variable of a custom struct type, which has exported methods with primitive arguments on it, these methods should be exposed to the MsgCall API in the following way:

package rlm 
// pkgpath is gno.land/r/leon/rlm

type Foo struct {
	num int
}

var MyFoo = &Foo{ num: 0 }

func (f *Foo) UpdateNum(newNum int) { // should be exposed
	f.num = newNum
}

func (f *Foo) unexportedMethod() { // should not be exposed
	...
}

func (f *Foo) ExportedComplexArgMethod(slice []*int) { // should not be exposed
	...
}

Users should be able to call the UpdateNum method like so:

gnokey maketx call -good-flag-name "gno.land/r/leon/rlm.MyFoo.UpdateNum" -arg "2" 

while not being able to call MyFoo.unexportedMethod() & MyFoo.ExportedComplexArgMethod().

The other issue is how to manage flags in gnokey; currently, the pkgpath and func flags are separate. Another semantic issue is that there is a distinction between functions and methods. Possibly, this can be solved with an -endpoint flag, which could have this usage:

gnokey maketx call -endpoint "gno.land/r/leon/rlm.MyFoo.UpdateNum" -arg "2"

I am not sure that this is the right way to use it in gnokey, so I am open to other suggestions.