store SM2 precomputed basepoint table in source

[emmansun]

Store the precomputed P-256 basepoint table in source rather than computing it at runtime, saving ~88kB from the heap. The flip side is that this increases binary sizes by ~77kB.

Reference the changes of

• crypto/elliptic: store P-256 precomputed basepoint table in source
• crypto/elliptic: use a const string for precomputed P256 table

[emmansun]

v0.5.0