Revamp private/public functions

Problem: having public as the default makes it very easy to accidentally
export local function by forgetting the `private` modifier.

Solution: functions are private by default and must be declared as `entrypoint`s
to be exported. So `entrypoint foo() = ...` instead of `function foo() = ...`.

We still accept the `private` modifier although it is redundant.

src/aeso_compiler.erl

@@ -253,7 +253,7 @@ insert_call_function(Code, Call, FunName, Args, Options) ->
         [ Code,
           "\n\n",
           lists:duplicate(Ind, " "),
-          "stateful function ", Call,"() = ", FunName, "(", string:join(Args, ","), ")\n"
+          "stateful entrypoint ", Call, "() = ", FunName, "(", string:join(Args, ","), ")\n"
         ]).
 
 -spec insert_init_function(string(), options()) -> string().
@@ -263,7 +263,7 @@ insert_init_function(Code, Options) ->
     lists:flatten(
         [ Code,
           "\n\n",
-          lists:duplicate(Ind, " "), "function init() = ()\n"
+          lists:duplicate(Ind, " "), "entrypoint init() = ()\n"
         ]).
 
 last_contract_indent(Decls) ->