box: export registered functions in box.func folder

Needed for #4182, #1260

@TarantoolBot document
Title: Export registered functions to box.func folder

Now all registered with box.schema.func.create functions are
exported in box.func folder.
Each function have :call and :drop method. The :drop method
just a shortcut for box.schema.func.drop interface.
The :call method is similar to net.box connection:call method
and allows to call a registered function directly.
All access checks are performed on each function call.

Example:
function sum(a, b) return a + b end
box.schema.func.create('sum')
box.func.sum
---
- language: LUA
  setuid: false
  name: sum
  id: 2
...
box.func.sum:call({1, 3})
---
- 4
...
box.func.sum:drop()