Allow to set breakpoints on function definition

If you set a breakpoint on a `def` line, it only breaks there when the function is defined. But this is confusing, because the debugger goes to that line every time you step into that function. So it would be nice if a break point on a def line also caused the debugger to break whenever that function is called.

The issue names no files or tests. Start by tracing how breakpoints on def lines are recorded and how stepping into a function is handled, then inspect existing debugger tests. Done means a breakpoint set on a function definition stops when that function is called, rather than only when the definition executes.

Written by the indexing model from the issue text.