Struct casting not supported by LLVM, suggest+implement struct pointer casting

[kyegupov]

A better error message for #161, to catch the problem before the LLVM verifier does.

[kyegupov]

Whoops, this needs more work: turns out struct pointers do not get casted either

[aykevl]

I think it's fine to always bitcast pointers. A superfluous bitcast is trivially optimized away, and it might make the code a bit simpler and more generic (think of double indirection for example).

[kyegupov]

Done.

[deadprogram]

Hi @kyegupov I switched this branch to point to the dev branch, which is the new active development branch. However I see that the tests are failing from your last commit.

[kyegupov]

@deadprogram done

[aykevl]

@kyegupov are you planning to fully implement this feature - that is, by extracting all values from the struct and creating a new struct with the new values? If not, I can merge this PR but will likely implement that myself.
I think such splitting/joining will be optimized away entirely, as LLVM tries to break up aggregates anyway to improve other optimization passes. And if not, slow code is always better than code that fails to compile.

[kyegupov]

Sorry, I'm no longer working on the project that required tinygo, so I'm not likely to invest more effort into this.

[aykevl]

Closed by #258.