Use TracePoint.allow_reentry to support TracePoint events while eval

[marshall-lee]

• Update Ruby and gems versions (needed to run a new test case using TracePoint API on new Rubies)

• Undefine alloc funcs of Context and ThreadsTable. This is to suppress a warning of this kind on modern rubies:

  warning: undefining the allocator of T_DATA class Byebug::ThreadsTable
  warning: undefining the allocator of T_DATA class Byebug::Context
  

• Use TracePoint.allow_reentry to support others' TracePoint events while evaling.

• Temporarily shut Byebug's TracePoint event handlers in the thread that issued allowing_other_threads { ... } to avoid infinite recursion in event handlers. CTX_FL_IGNORE flag was useful here.

• Add a test case using TracePoint.new(:class) hook inspired by @fxn's comment https://github.com/deivid-rodriguez/byebug/issues/564#issuecomment-497499214

Fixes #564