MIR-borrowck: print named fields with `.name` rather than `.<num>`

[pnkfelix]

The MIR-borrowck printing of Lvalues visible to end-users prints accesses to a named struct/enum field with a description like obj.3 instead of obj.field.

This needs to be fixed before MIR-borrowck is usable in real-world.

(And fixing it sooner would probably make developing MIR-borrowck a bit more enjoyable.)

[pnkfelix]

@zilbuz tells me that they are working on this.