Move amount closer to currency

[dudo50]

Currently we have separate .amount parameter in builder, but this is very far from symbol selection and when selecting multiple assets it could be confusing to input two numbers into .amount.

Remove .amount parameter from builder and move it closer to .currency for more user friendly amount specification.

.currency(multiasset[{symbol: WUD, amount: 2101293}, {symbol: DED, amount: 18312}, ])