Support % slippage limit

[mrq1911]

toTx currently expects only limit as absolute amount, would be useful to allow also {slippagePct: '1'} for router to calculate limit from context automatically to reduce errors, also it should be possible to not define limit at all - in that case the default (f.e. 1%) would be applied

these cases should be supported:

• .toTx({limitPct: 1})
• .toTx({limit: 123.33})
• .toTx(123330000) (legacy support)