struct/union/enum about saltwater HOT 5 CLOSED

Error is different since 5b2bcaf:

struct s { int i; }
thread 'main' panicked at 'not yet implemented: struct and union members', src/parse/decl.rs:578:9

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Enums are done since ddc165c!

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Struct and union declarations are done since 44a13ce. Need to implement expressions and codegen.

struct s { int i; } s;
int main() {
        return s.i;
}
thread 'main' panicked at 'not yet implemented: struct and union members are very much not implemented', src/parse/expr.rs:706:33

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Possible ways to implement this:

Have a cache in ir::LLVMCompiler that stores offsets into structs. For unions, the offset is always zero. The size of a struct is the greatest offset + sizeof(member at that offset). Accessing a struct member is fetching *(s + offset).

Ways to calculate the offset: For now, just do sizeof(member) + (8-sizeof(member))%8, i.e. round up to the nearest multiple of the alignment.
In the future, sort members from largest size to smallest, excluding arrays. Put chars, shorts in spare bytes. Prohibited by the C standard, see section 6.7.2.1.

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Done in 01f53f2!!!

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