Give helpful error message (and auto-fix in formatter) when using `=` in record literals

[rtfeldman]

When using = instead of : in record literals (including record updates), we should do both of these:

• Give a helpful error message
• Have the formatter automatically change it to a :

[rtfeldman]

I'm not sure how tricky it would be to get the parser to recognize = in record literals instead of : given how special-cased = is due to patterns in defs...does this seem like a feasible idea @joshuawarner32?

[joshuawarner32]

At a casual glance, I don't see anything in the record parsing code that would prevent this.

From a backwards-compatibility perspective, I would suggest not actually considering records with '=' as valid code (i.e. always emit a [non-fatal] error, and always fix in the formatter) - that way if we want to repurpose '=' to mean something else in that position, that option remains available.