Make ninjaString an interface

There are 8935901 *ninjaString objects generated in an AOSP
aosp_blueline-userdebug build, and 7865180 of those are a literal
string with no ninja variables.
Each of those *ninjaString objects takes a minimum of 48 bytes for
2 slices, plus 8 bytes for the pointer to the ninjaString.  For
the literal string case, one of those slices has a single element,
(costing another 16 bytes for the backing array), and the other
slice is empty, for a total of 72 bytes.

Replace *ninjaString with a ninjaString interface.  This increases
the size of the reference from 8 bytes to 16 bytes, but using
a type alias of a string for the literal string implementation uses
only 16 bytes, saving 40 bytes per literal string or 314 MB.

Test: ninja_strings_test
Change-Id: Ic5fe16ed1f2a244fe6a8ccdf762919634d825cbe