[smklein]

Node::property returns a PropValue for callers to pattern-match, which
suits properties of unknown type, but callers that know the type ended up
writing a three-arm match around every read.

• Node::property_i32 / property_u32 / property_i64 / property_u64 /
  property_f64 / property_string / property_fmri, one per scalar
  topo_prop_get_* function, each returning Result<T, Error>.
• A missing property surfaces as Error::PropertyNotFound { group, name }
  and a type mismatch as Error::PropertyType { group, name, expected },
  mapped from libtopo's ETOPO_PROP_NOENT / ETOPO_PROP_TYPE. Callers
  typically log the two differently, and the old Error::Topo(String)
  only let them tell the cases apart by message text. Node::property
  reports a missing property the same way; other codes stay Error::Topo.
• The fixed-size getters share one generic helper whose function-pointer
  parameter ties the Rust out-type to the C getter that writes it.
• Two private unsafe helpers, adopt_topo_string and adopt_topo_nvlist,
  own the copy-then-topo_hdl_strfree and cast-then-OwnedNvList sequences.
  The new getters and the existing label, resource, asru, fru,
  fmri_parse, fmri_to_string, snapshot uuid, property, and
  property_groups paths all go through them.
• Array getters are not wrapped; PropValue already covers them.
• libtopo bumps to 0.2.0, since Error is not non_exhaustive and the new
  variants can break exhaustive matches.

Tested on a Helios 3 host: fmt, clippy -D warnings, and the full test
suite under pfexec all pass.

Fixes #16