Fix UFix64 <-> UInt256 conversion

[sisyphusSmiling]

Problem

Current behavior simply truncates decimal which is poor user experience, particularly for dust values. In the edge where the total supply of a token is 1 (prior art here in memecoins), users wouldn't be able to bridge any value and worse, if attempted the request would succeed in escrowing the value only to return none on the receiving end.

Acceptance Criteria

Conversions are appropriate for the given amount. E.g. 0.001234 converts to 12340000000000 for decimals == 18. Any implementation should also cover the case where an ERC20's decimal value is less precise than Cadence's UFix64 decimal value of 8, rounding down to prevent overage in either direction.

[sisyphusSmiling]

Reopening as new bugs were uncovered

[sisyphusSmiling]

Reopening after uncovering another issue. @bluesign shared a fix - would you be able to submit a PR or would you mind if I do on your behalf?