ImmutableCString is a string class similar to CString except it's
immutable. ImmutableCString occupies 16 bytes instead of 8 for CString.
However, ImmutableCString is able to avoid memory allocation when
constructed from a string literal, and in that way it us similar
to StaticString.
ImmutableCString can be auto converted from StaticString.
The backend tag tracking is updated to use ImmutableCString and
the FrameGraph resource manager us updated to use StaticString.
Together these changes significantly cut down heap allocations due to
internal tagging.
We also add optional tracking to {Immutable}CString.
