viml/parser/expressions: Start creating expressions parser

Currently supported nodes:

- Register as it is one of the simplest value nodes (even numbers are
  not that simple with that dot handling).
- Plus, both unary and binary.
- Parenthesis, both nesting and calling.

Note regarding unit tests: it stores data for AST in highlighting in
strings in place of tables because luassert fails to do a good job at
representing big tables. Squashing a bunch of data into a single string
simply yields more readable result.

test/unit/eval/helpers.lua

@@ -1,5 +1,6 @@
 local helpers = require('test.unit.helpers')(nil)
 
+local ptr2key = helpers.ptr2key
 local cimport = helpers.cimport
 local to_cstr = helpers.to_cstr
 local ffi = helpers.ffi
@@ -91,10 +92,6 @@ local function populate_partial(pt, lua_pt, processed)
   return pt
 end
 
-local ptr2key = function(ptr)
-  return tostring(ptr)
-end
-
 local lst2tbl
 local dct2tbl