vhpi: Add support for getting record types by name

vhpi: Add support for getting record types by name

When cocotb tries to access an object with a name like `VHPI5.O.F`, it
doesn't know whether O is a signal or a record. This ambiguity could be
resolved by trying to look up the name like `:VHPI5:O.F` or
`:VHPI5:O:F`, but cocotb can't determine which to use ahead of time.
This is a limitation of cocotb, but probably not one which will ever get
resolved.

Add support for looking up record types by name, primarily to allow
cocotb to access them.