fix(bootstrap): type compatibility fixes for self-hosting

- Allow int32 <-> int and uint32 <-> int/uint comparisons
- Allow uint32 <-> int32 comparisons
- Allow String indexing (returns char8)
- Allow *char8 -> String assignment (C string literal to Bux String)

These fixes allow buxc2 (self-hosted compiler) to build successfully.
All 14 modules compile; buxc2 binary is 111KB.

src/sema.nim

@@ -883,6 +883,8 @@ proc checkExpr(sema: var Sema, expr: Expr, scope: Scope): Type =
       return obj.inner[0]
     elif obj.isPointer:
       return obj.inner[0]
+    elif obj.kind == tkStr:
+      return makeChar8()
     else:
       sema.emitError(expr.loc, "cannot index non-slice/non-pointer type")
       return makeUnknown()