[DMS-47] Support text type

[DK318]

This PR adds support for text in motoko-san. This is done by adding a field for each string literal and mapping them to field access.

Example

let x = "some_string";

Translation

...
field $text: Int // was generated for "some_string"
...
define $Perm($Self) (true && .. && acc($Self.$text, write)) // string literal could be accessed in any scope of the program
...
var x: Int;
x := $Self.$text;

[GoPavel]

LGTM except small nitpicks