Fix data races with sync package types

[mattn]

Two data races reported in #363, both visible with go test -race ./vm/ in TestPackagesSync:

• sync package types were registered as value types, so every VM access copied the lock-containing struct through reflect while goroutines mutated it. They are now registered as pointer types (as suggested in the issue), which also fixes the copylocks misuse itself.
• anonCallExpr wrote to the shared AST via a self-assigning SetPosition on every call, racing with concurrent execution of the same AST. The call was a no-op and is removed.

The full test suite now passes under -race.

Fixes #363