Discuss tradeoffs involved when using static vs dynamic tables

[dvdplm]

See https://github.com/paritytech/libsecp256k1/pull/41#issuecomment-605059456

Give users enough info to decide whether they should use static or dynamic tables, e.g. things like:

• binary size
• runtime memory (?)
• cost of table generation
• …

[sorpaas]

Users can now choose themselves static-context vs lazy-static-context.