This change adds Int.ModSqrt to compute modular square-roots via the
standard Tonelli-Shanks algorithm, and the Jacobi function that this and
many other modular-arithmetic algorithms depend on.

This is needed by change 1883 (https://golang.org/cl/1883), to add
support for ANSI-standard compressed encoding of elliptic curve points.

Change-Id: Icc4805001bba0b3cb7200e0b0a7f87b14a9e9439
Reviewed-on: https://go-review.googlesource.com/1886Reviewed-by: Adam Langley <agl@golang.org>