[WIP] Tool slither-format: automatic code improvements

[rajeevgopalakrishna]

This PR (depends on #237 and #236 and replaces PR #235 which was created against a stale dev branch by mistake) addresses #150 to add a new utility tool slither-format which uses slither detectors to identify code patterns of concern (w.r.t security, readability and optimisation) and automatically fix those code patterns with suggested changes.

The current list of detectors used to detect and fix include: unused-state, solc-version, pragma, naming-convention, external-function, constable-states and constant-function.

Detectors highlight names, context and source-mapping of code constructs which are then used by slither-format to programmatically locate those constructs in the Solidity files and then replace them with changes based on best practices. Lexical analysis for identification of such constructs is confined to the smallest possible region to avoid conflicts with similarly named constructs (with potentially different types or signatures) in other scopes, functions or contracts within the same file (because of shadowing, overloading etc.).

Done: Framework and basic functionality. Unit testing.
WIP: More real-world testing and bug-fixes. More tool options.
Known bugs:

1. naming-convention formatting doesn't work on NatSpec comments, e.g. @param.
2. Variables used as indices on LHS (e.g. _to in balances[_to] = 100) don't get reported from Slither in variables that are read and hence are not formatted in naming-convention.
   3. Bugs potentially from incorrectly reported source mappings, which might be related to #218.

[montyly]

Few recommendations:

• Consider using the cli crytic_compile options, for example:
  

  slither/utils/upgradeability/__main__.py

  Line 6 in 7cadf98

  from crytic_compile import cryticparser

  
  

  slither/utils/upgradeability/__main__.py

  Lines 31 to 38 in 7cadf98

  	cryticparser.init(parser)
  	if len(sys.argv) == 1:
  	parser.print_help(sys.stderr)
  	sys.exit(1)
  	return parser.parse_args()

  
  As a result,
  

  slither/utils/slither_format/__main__.py

  Line 54 in 2e93076

  slither = Slither(args.filename, is_truffle=os.path.isdir(args.filename), solc=args.solc, disable_solc_warnings=True)

  
  Would be become slither = Slither(args.filename, **vars(args)) , and this flag can be removed:
  

  slither/utils/slither_format/__main__.py

  Line 30 in 2e93076

  parser.add_argument('--solc', help='solc path', default='solc')

• Consider adding slither-format to setup.py, so we can call slither-format directly from the command line