Make Scilla's OCaml API more convenient to use

[anton-trunov]

We've been developing a number of tools using Scilla as an OCaml library and this experience has showing that we need to create more convenient OCaml API. For example,

• a function that takes a file name and library path and outputs a typed ASTs;
• a function that takes a procedure name (identifier) and returns the corresponding contract component;
• a function that takes an identifier and returns its type.

Right now we use something like this to get types / locations:

    let check_typ_warn s =
      let t = (ER.get_type (get_rep s)).tp in
      let lc = ER.get_loc (get_rep s) in
...