Fix #1469 (SET ROLE regression)

[lovasoa]

this fixes a regression that was introduced with the new SetRole statement

[lovasoa]

Hello !
@alamb , this fixes a regression that had a big impact on SQLPage, where it is common to name a variable role.
Initially reported in sqlpage/SQLPage#644 (reply in thread)

[lovasoa]

I changed the code to use maybe_parse, which is indeed much cleaner and resolves both your points. Thanks @iffyio !

[iffyio]

LGTM! cc @alamb

[lovasoa]

@alamb , the test failure does not seem to be related to the contents of the pr.

/home/runner/.cargo/bin/cargo-tarpaulin: error while loading shared libraries: libssl.so.1.1: cannot open shared object file: No such file or directory

[alamb]

Thanks @lovasoa and @iffyio

[lovasoa]

Thanks for merging, @alamb !