Do not throw on parseUnits if input truncatable

[coinwalletdev]

Currently, if you do the following:

require('ethers').utils.parseUnits('123.4560000', 4)

ethers.js will raise an exception while it should simply remove the trailing 0 from the string before converting it. This PR does this.

I understand the need to raise an exception in more ambiguous cases (e.g. parseUnits('123.45678', 4) - should you floor, round, ceil, ... ?), but if there is trailing 0, this is a trivial case.

[ricmoo]

I think this should still be an error, I think.

A value with significant digits (0 is a completely valid significant digit) is being asked to truncate those significant digits, and in general I would think this indicates an error somewhere upstream. I agree though, it has no meaningful effect on the value.

Can you give an example where this was an issue?

I'll ponder about it over night. :)

[coinwalletdev]

I'm not sure if it's up to the library to decide whether or not 0s have a meaning - IMHO as an author you should just choose sane default options for your users.

Deciding this for your users force them to reimplement by themselves roughly the parseFixed function which makes the whole point of parseUnits more or less useless.

Ideally you'd feed another option to parseUnits allowing the user to choose if they want a strict mode (throw an exception), floor the value, ceil the value, etc..

This is just my 2 cents and nevertheless it seems you know what you're doing with this amazing library :)

[ricmoo]

I was still a little on the fence, but after thinking about it, agree it makes sense. I don't error if you you include leading-zeros that would expand beyond a support numeric value in magnitude, so should probably not treat precision differently.

I've made this adjustment in 5.2.0.

Please try it out and let me know if you have any problems. Thanks! :)

[ChALkeR]

See #1974