Auto merge of #4602 - EthanTheMaster:issue-4001, r=flip1995
Add suggestion for mul_add
Issue #4001: Whenever `a*b+c` is found where `a`,`b`, and `c` are floats, a lint is suggested saying to use `a.mul_add(b, c)`. Using `mul_add` may give a performance boost depending on the target architecture and also has higher numerical accuracy as there is no round off when doing `a*b`.
changelog: New lint: `manual_mul_add`