v0.51 "The Great Summer Harvest", again

This legacy-only release introduces:

• Version() for better version and feature tracking.

And also fixed one issue found during testing.

1. fixed constant folding of various new operators (sin, cos, round, trunc, %)

While we're at it, also added the pi operator as it's trivial.

Again, please use the lexpr version if possible and the legacy version won't receive more feature updates.
lexpr version latest release is v0.60.