Returns a scope with some default functions and constants

@spec get_in(nil, key() | index(), map()) :: nil

@spec get_in(map(), key(), map()) :: nil | term()

@spec get_in(list(), index(), map()) :: nil | term()

@spec get_in(list(), key(), map()) :: nil | term()

Tries to get values from a subject.

Subjects can be:

• maps
• keyword lists
• lists (with integer keys)
• nil (special case so access to undefined variables will not crash execution)

Keys can be:

• strings
• atoms (you can pass a resolve map that converts the given atom key into a string key)
• positive integers (for lists only)

The Abacus parser will reference this function extensively like this:

a.b[2] will turn into

get_in(get_in(var0, :var1, var_lookup), 2, var_lookup)

in this case, var_lookup will be:

%{
  var1: "a",
  var2: "b"
}

Renames first-level variables into their respective :"var#{x}" atoms