Changes baseFee, reserveBase, reserveIncrement to XRPAmount

[movitto]

Addresses weird custom serialization of these fields, see
issues #3165 and #3502 on github.

Work in progress, submitting for initial feedback, will require
an amendment to transaction from the old representation to the
new.

Tests updated to pass.

[nbougalis]

This change, as is, would cause massive breakage. Implementing this properly would, at the very least, require an amendment (which you've pointed out), as well as support for "dual-type" fields (U64 prior to the amendment, Amount afterwards).

I don't mean to dissuade you from pursuing this, but I think that the cost-benefit ratio is simply not favorable.

[carlhua]

@movitto Thank you for your contribution! rippled can always use more contributions! However, like @nbougalis pointed out, the changes would be a breaking change. I am going to convert this PR to draft as we iterate on this.

[nbougalis]

@movitto, I'm going to close this PR. Please feel free to reopen it at a later time if you decide to pursue this further. 🖖