Pancake shape improvement

It seems that currently we can read from shape by index, but cannot write to shape by index. For example
```c
fun main () {
var hello = lds {3} @base;
// hello.0 = 0; --> this doesn't work
stw @base, 0;
return 0;
}
```
Would be nice to write to shape by index as well (if can we can index by namestrings that'd be even better!)