v0.3.0

Editor support for xphp v0.3.0, plus on-save whole-project diagnostics,
cross-project navigation, and hover fixes.

• xphp 0.3.0 language features: out/in variance, Closure(int $x): bool
  signature types, extended turbofish, method-level generics, new diagnostic codes
• On-save authoritative diagnostics — runs the compiler's own check() in-process
  to catch call-argument closure conformance and cross-file generic errors the
  live pass can't
• Multi-root navigation — open a file from a sibling project and its symbols
  resolve for go-to-definition, references, completion, and rename
• Hover fixes — correct generic method signatures (map(…)) and flow-sensitive
  variable types

What's Changed

• 0.3.0 by @math3usmartins in #13

Full Changelog: v0.2.5...v0.3.0