rusti: disable unused variable warnings

[kud1ing]

Say, you're prototyping some code or doing some quick calculations, those warnings are not helpful:

rusti> let x = 42;
<anon>:35:4: 35:7 warning: unused variable: `x`
<anon>:35 let x = 42;
              ^~~
()
rusti> let y = x + 42;
<anon>:37:4: 37:7 warning: unused variable: `y`
<anon>:37 let y = x + 42;
              ^~~
()
rusti> y
84

[infynyxx]

+1

[catamorphism]

#3266 will allow unused variable warnings to be disabled or enabled with attributes, as desired.