New Lint: Avoid too many static code paths

Based on this HIC++ guideline, in addition to the existing cyclomatic complexity lint. It basically comes down to the possible number of branches in a function's code path, not counting loops. E.g. this is okay:

if this { that }
else { or_that } // Just 2 code paths

Wherease this is not so cool:

if this1 { that1(); } // ... 2 paths (do or don't)
if this2 { that2(); } // ... 4 paths
if this3 { that3(); } // ... 8 paths
...
if this10 { that10(); } // ... 1024 paths - Have fun with unit testing!

Start by locating Clippy's existing cyclomatic complexity lint and reading how similar lints are structured. Use the examples in this issue to define the expected static-path behavior; the work is done when the new lint handles these cases and has suitable coverage.