Initial attempt to add ON UPDATE support.

[EmmaLouise2018]

Opening up this PR, because I am not entirely sure how to completely resolve this issue and would be grateful for some input/suggestions.

What problem does this PR solve?

This PR was opened in reference to #280 in an attempt to add support fo ON UPDATE sql statement

What is changed and how it works?

The change was to try to update the grammar to support ON UPDATE clauses without ON DELETE clauses. The changes are NOT complete.

Running make test generates a parser.go file, which when the tests are run they all pass. In addition, all the sql statements I am working with seem to parse correctly. However, the change I am attempting to make also introduces, perhaps unsurprisingly reduce/reduce conflicts that I am not sure how to resolve.

This is my first time working with Yacc, and it's been a while since I've defined a grammar. Would be grateful of any input/suggestions on how to resolve these conflicts or if there is a better approach for how to add this support in the first place.

Side effects

This is currently introducing reduce/reduce conflicts I am unsure how to resolve.

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[EmmaLouise2018]

This issue is better resolved in the following pr #331