- renamed core.d to main.d due to conflicts
- modified source code from "Datastructures and algorithms in D" to match our needs:
- added string_literal.d for parsing of string literals
- added identifier.d for parsing of identifiers
- modified lexer.d to enable lexing of string literals and identifiers and to disable lexing of operators
- modified precedence_climbing.d to parse expressions with identifiers instead of operators and function calls -> in future we need a method to register certain identifiers as operators