Use `dyn.InvalidValue` to indicate absence

[pietern]

Changes

Previously, the functions Get and Index returned dyn.NilValue to indicate that a map key or sequence index wasn't found. This is a valid value, so we need to differentiate between actual absence and a real dyn.NilValue. We do this with the zero value of a dyn.Value (also captured in the constant dyn.InvalidValue).

Tests

• Unit tests.
• Renamed Get and Index to find and update all call sites.

[pietern]

Confirmed the integration tests are good as well.