Make 'did-you-mean' facility better

[kjcjohnson]

This pull request introduces the following changes:

• The did-you-mean (DYM) facility now considers functions and variables/constants separately
• The DYM facility now handles locally-bound variables
• Improvements to the DYM edit distance algorithm
• Now, only the top three candidates are returned
• The DYM facility checks for function arity and, if no matches, falls back to all in-scope functions
• Error message deduplication, so only one error will be printed at the source instead of also at intermediate nodes
• Fixes a calculation error with identifier positions

Closes #59 and #58.