error, solver: cleanup error var names

solver, error: remove dependency on 'fmt', and expose error constants

solver, math: fix artificial variable expr terms sharing the same memory as the term being optimized away, add clone methods to Expr and Constraint, speed up solveFor() by not performing any operations if the coefficient is 1

math, readme: update symbols to unsigned 64-bit integers, and update remark in readme

solver: if a variable is already marked as an edit variable, ignore it

readme: format

readme: format

readme: add remark on how symbols are created

solver, math, math/test: have symbols be uint32s instead of pointers that are bit-masked with their kind, rename symbolType to SymbolKind, rename zero to eqz, reduce bit size of some data types, update benchmarks

readme: fix small comment typo

math, solver, solver/test, readme: use pointers for symbols instead of integers to allow gc to dispose of unused symbols, allow for variables to created detached from a solver, and update tests and readme to reflect new method for instantiating symbols

math, solver, solver/test: allow for constraints to be deleted (supported with tests), delete zero-coefficient terms in addSymbol, remove Row and move priority to Tag, internally rename rows to tabs to represent the solvers tableau

solver: add cassowary test for the use case of padding a ui

solver: remove old refactoring comments, rename priority to prio for Row

math, solver, readme: modify tests to demonstrate api usage, update readme with more complex example, rename pkg to casso