Improved costing, new operations, new context properties

• new and more general costing algorithm
• optimized execution of cost formulas
• new elliptic curve for GroupElement operations
• new context properties (see special.sigma.Context trait)
  • dataInputs - A collection of inputs of the current transaction that will not be spent
  • headers - a fixed number of last block headers in descending order (first header is the newest one)
  • preHeader - fields of block header with the current spending transaction, that can be predicted
    by a miner before it's formation
• new operations for authenticated trees (see special.sigma.AvlTree trait), including chained compositions
• proveDlog, proveDHTuple can accept any expression as arguments
• def f(x: Int): Int declarations of functions
• various internal improvements in type checker, compiler, byte code interpreter etc.