Add test:valgrind rake task and use it in CI

[dylanahsmith]

To help with detecting memory leaks, I've added a test:valgrind rake task and used it in CI.

• bin/fmt was successfully run

[dylanahsmith]

Looks like Rotoscope.new has a leak, but the underlying leak is in ruby's rb_tracepoint_new. Specifically, it uses RUBY_TYPED_NEVER_FREE as its data free function, even though the data itself is dynamically allocated through TypedData_Make_Struct ​(https://github.com/ruby/ruby/blob/v3_0_2/vm_trace.c#L733)

[jahfer]

Great find on the leak!