Implement `textDocument/references` for finding references to the symbol under the cursor

[mfelsche]

This feature resolves a list of references to the symbol under the cursor.

See the spec: textDocument/references.

One possible implementation could generate a mapping from all possible symbols that refer to something (tk_fletref, tk_varref etc.) to the AST node / symbol they refer to, and from that node to an Array[AST] which holds all the references to the definition symbol.