Add peek functionality to the queue_pkg

[dalex78]

Objective:
Add a peek functionality to the queue_pkg: returns the element at the front the queue. It does not deletes the element in the queue (contrary to the already existing pop command).

Rational:
I intend to use this feature to improve the AXI(L) verification components (I understand from issue #726 that the development of VC is in stand-by, any news regarding that ?).

What has been done:

• The peek functionality has been implemented here: peek_functionality
• I have launched the following tests on Modelsim (Interl FPGA Starter Edition 2020.1): all run.py inside the vunit/vhdl architecture:
• • Some of the tests of the verification_components have failed but are not related to this work (see issue vcomponents tests are failling with ModelSim #642).

Remaining work: (if this feature is wanted/accepted by the community)

• Launch all tests with tox on modelsim (currently doing that),
• Test with other simulators,
• Update the documentation

Additional remarks:

• I think the codec_builder_pkg and queue_pkg files deserve a little bit of cleanup: more documentation, removing hard-coded values, etc (without breaking compatibility). I can work on that before doing a pull request, or it could be done at a later time.

This is my first (code) contribution, don't hesitate to give feedback.

[LarsAsplund]

Agree that a peek is useful. I thought about if when fixing #642. Note that we have something similar in com (

vunit/vunit/vhdl/com/test/tb_com.vhd

Line 937 in 4a0e258

elsif run("Test peeking at messages in a mailbox") then

)

You're right, the facelift of all VCs has been on hold for a while. The last time I worked on it I presented the standard rules in the Gitter channel and the feedback was such that we can proceed. There is one thing that I will revisit and maybe do a bit differently. This is also a backward compatibility breaking update so it we need to be a bit more careful when deciding on when to introduce it.

I suggest you go ahead and make a PR of peek for queues.

[dalex78]

Corresponding PR: #814