Issue #2095 - Provide constants for e.g. the zero address and max uint256

[ernestosperanza]

What was wrong?

If I need to compare an address returned from a contract or variable to the zero address or want to e.g. use the maximum allowed uint256, I need to define those myself. (issue #2095)

How was it fixed?

I added the constants.py module and created constants for the following commonly used values:
As strings: ADDRESS_ZERO, MAX_INT, HASH_ZERO
As integer: WEI_PER_ETHER

I replaced the hardcoded values in the codebase with the new constants.
And write a little doc for the constants.

Todo:

If is required I can replace the hardcoded values in docs two, I didn't want to change documentation without permission.

Let me know if there's any improvement that I can apply, thanks :)

[kclowes]

Thanks for the PR @ernestosperanza! It looks like the core and ethpm tests are failures that we have in master, but the docs and the lint tests are failures related to this PR. Let me know if you want help fixing those!

[ernestosperanza]

@kclowes let me do some documentation reading and follow all the instructions, then if I'm still having problems I will get back to you, thanks

[ernestosperanza]

@kclowes I'm still getting errors in the docs but I think that those belong to the ethpm.rst, let me know If I have to modify anything, thanks in advance

[kclowes]

I just rebased the current master and pushed. Hopefully that will fix it!

[kclowes]

@ernestosperanza - Looks like there are a few real failures where ADDRESS_ZERO needs to be swapped for HASH_ZERO

[ernestosperanza]

Sorry, my bad, now should be everything ok.
Let me know if I need to edit any detail, thanks!

[kclowes]

Thanks @ernestosperanza! Looks good, I just left one nitpick!

[kclowes]

Looks good to me! Thanks @ernestosperanza!

[ernestosperanza]

Let me know how I can still helping to the proyect, the experience was really enriching for me

[kclowes]

Let me know how I can still helping to the proyect

Great! We love contributions! Any of the issues tagged with the "Good First Issue" are usually the best to start with since they don't require a detailed knowledge of the codebase, so keep an eye out for those. We also will need help converting methods to async, but I haven't put any issues up yet. I'll do that soon!