Use --Wshadow

[tfpk]

This may be contentious, but I think it makes a lot of sense to add -Wshadow as a compilation flag. Students often cause strange issues that are hard to trace by shadowing variable names. and as far as I can tell there is no good reason for us to allow them to do so. If they're shadowing i or j, they're too deeply nested. If they're shadowing anything else... surely they can name things better, or keep a temporary copy.

Is there any reason a 1511 student (or a 1521/2521 student for that matter) would want to shadow a variable?

[andrew-taylor]

Added in e2cc9a2

Thanks!