The following AST avoids unnecessary derefs now:
* error set decl
* field access
* array access
* for loops: replace ensure_indexable and deref on the len_ptr with a
special purpose ZIR instruction called indexable_ptr_len.
Added an error note when for loop operand is the wrong type.
I also accidentally implemented `@field`.
In rare occassions, it may happen that string table is allocated free
space preceeding symbol table. This is an error in the eyes of the `dyld`
dynamic loader and thus has to forbidden by the linker.
