Caret diagnostics improvements

[alexrp]

We have fairly reasonable caret diagnostics in master now, but there is still some work to be done:

• Allow passing multiple locations to the logger (currently allows only one).
• Look source code up in the in-memory list of files instead of reading from disk.
• Allow disabling caret diagnostics (perhaps have a FLECT_DIAGS environment variable).

Not super important, but nice to have if practical:

• Mark the entire token relevant to a particular message.
• Colorize the token relevant to a particular message (e.g. the token would be red for errors).

[alexrp]

We now support multiple locations in caret diagnostics. See commits 26640fb and 9900451.