b03c24db8c Add a checked version of weight mul fee_rate (yancy)
Pull request description:
Add a checked version of fee_rate * weight. While I like the trait version of just being able to multiply `feerate * weight`, it's not really very useful imo since a large input feerate could cause an overflow. Instead of changing the trait in https://github.com/rust-bitcoin/rust-bitcoin/pull/1849 (not idiomatic enough I guess) I added a `checked_weight_mul` method to `FeeRate`.
ACKs for top commit:
apoelstra:
ACK b03c24db8c
Kixunil:
ACK b03c24db8c
Tree-SHA512: 231ade94291becadcea9ea2a40a5daf96b77f01a29cca2494d7bbe4f7de5b412fa8fc816ea249268569f5378410185d9349fd687533bf3a422a752997e107a2b
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