Add needles "current_function", "called_from", and "return_value" to
ReturnStmt description.
Add IndexedString methods:

lookupPrimaryIndexAtCharacter(), which gets an iterator to the needle lookup
entry for the primary index at a given character position, or the end of the
needle lookup if no index exists at that position.

getPrimaryIndexAtCharacter() returns the string key of the primary index at a
given character position, or an empty string if no index exists at that
position.
Add class seec::clang_epv::CharacterLinks, which describes linking information
for a single character in an explanation, including information about the
primary index at that character, and any Decl or Stmt associated with that
index.

Add Explanation::getCharacterLinksAt(), which returns a CharacterLinks object
describing the linking information for the character at a given position.