Replace usage of our own vector/xvector classes with DRuntime's

[AndrejMitrovic]

Once our minimum supported version becomes v2.088 (including LDC), we can leverage the DRuntime bindings for vector/string classes: https://dlang.org/changelog/2.088.0.html#std_vector

This shouldn't be done yet as we are still using older versions of DMD (and LDC).