Provide better error message when assigning to tuple element

[rdmello]

Description

This change addresses the second part of #1560 by adding an explicit error message when assigning to a tuple element:

./src/bpftrace -e 'i:s:1 { $x = (1, 2); $x.1 = 1; }'
stdin:1:22-30: ERROR: Tuples are immutable. Consider creating a new tuple and assigning it instead.
i:s:1 { $x = (1, 2); $x.1 = 1; }
                     ~~~~~~~~

I went with the approach of making this a parse-time error for simplicity. I ran all the unit tests and verified that the number of shift-reduce conflicts haven't changed after implementing this.

An alternative approach is to allow tuple assignment to parse successfully, and then make tuple assignment error later during semantic analysis. This would be a little more complex, but would also provide a better error message (since it can disambiguate between assigning to tuples vs. assigning to maps). Let me know if this approach is preferable.

Checklist

• Language changes are updated in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[danobi]

LGTM, but can you add a couple more tests for other conditions?

eg (1,0).1 = 0, ((1,0),3).0.0 = 3, ((1,0),3).0 = (0,1). You can just check that parsing failed for these, no need to check error message.

[danobi]

Oh, also update the changelog, please.

[rdmello]

@danobi Thank you for reviewing this...I added a few more tests and updated the changelog.

[danobi]

Thanks for the PR!