solidity optimizations (#781)

* gas optimisation and private constant explicit

* use constants for the well known static values

* use constant 64 byte zero buffer in expand_msg

* use full object XOR vs per byte

* remove useless unchecked block

* use return variables on functions vs. declaration

* use iszero and unchecked on simple math

* use less memory vars and leave notes on overflow

* extend unchecked behaviour + missing optimisation

* readding floating pragma for compilation

* forward gas without 2000 subtraction

* fix: fixed missing argument in compute_ith_lagrange_at_z

* fix: get rid of pragma experimental AbiEncoder

* streamline msg expansion

* combine loops for efficiency

* combine expand into hash_fr

* loop optimisation

* missed commits - 500 gas

* remove concat and use items independently

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Co-authored-by: thedarkjester <[email protected]>