refactor(setSpace): fix typing

[louh]

Fixes TypeScript errors in setSpace().

The main issue is this: setSpace accepts a three-letter string, where every three-letter combination of be statically checked by TypeScript. However, when running a substring(), the return type is a string, rather than one of the known letters, and keeping the return value a string literal is better for type safety.

After extensive research(!!!) it seems like the solution is just to say .substring() as MyType, because inputs are already typechecked and we just need to tell TypeScript that after this point the only valid values for substring() are values we know about, don't worry about it.

Also removes a code path for a position value of k which is never used and does the same thing as h.