Added 'lenient ops' module (#6754)

* added 'lenient ops' module

* made lenient ops inline/noSideEffect; updated copyright

* moved tests into external module

* added warning for the '==' operator

* mentioned lenientops in the standard library overview

* removed test on nimout -- too fragile

* removed `==` and moved doc link to core section

tests/float/tlenientops.nim

@@ -0,0 +1,100 @@
+discard """
+  exitcode: 0
+  output: ""
+"""
+
+import lenientops
+
+proc `~=`[T](a, b: T): bool = abs(a - b) < 1e-7
+
+block: # math binary operators
+  let i = 1
+  let f = 2.0
+
+  doAssert i + f ~= 3
+  doAssert f + i ~= 3
+
+  doAssert i - f ~= -1
+  doAssert f - i ~= 1
+
+  doAssert i * f ~= 2
+  doAssert f * i ~= 2
+
+  doAssert i / f ~= 0.5
+  doAssert f / i ~= 2
+
+block: # comparison operators
+  doAssert 1.int < 2.float
+  doAssert 1.float < 2.int
+  doAssert 1.int <= 2.float
+  doAssert 1.float <= 2.int
+  doAssert 2.int >= 1.float
+  doAssert 2.float >= 1.int
+  doAssert 2.int > 1.float
+  doAssert 2.float > 1.int
+
+block: # all type combinations
+  let i = 1
+  let f = 2.0
+
+  doAssert i.int + f.float ~= 3
+  doAssert i.int + f.float32 ~= 3
+  doAssert i.int + f.float64 ~= 3
+  doAssert i.int8 + f.float ~= 3
+  doAssert i.int8 + f.float32 ~= 3
+  doAssert i.int8 + f.float64 ~= 3
+  doAssert i.int16 + f.float ~= 3
+  doAssert i.int16 + f.float32 ~= 3
+  doAssert i.int16 + f.float64 ~= 3
+  doAssert i.int32 + f.float ~= 3
+  doAssert i.int32 + f.float32 ~= 3
+  doAssert i.int32 + f.float64 ~= 3
+  doAssert i.int64 + f.float ~= 3
+  doAssert i.int64 + f.float32 ~= 3
+  doAssert i.int64 + f.float64 ~= 3
+  doAssert i.uint + f.float ~= 3
+  doAssert i.uint + f.float32 ~= 3
+  doAssert i.uint + f.float64 ~= 3
+  doAssert i.uint8 + f.float ~= 3
+  doAssert i.uint8 + f.float32 ~= 3
+  doAssert i.uint8 + f.float64 ~= 3
+  doAssert i.uint16 + f.float ~= 3
+  doAssert i.uint16 + f.float32 ~= 3
+  doAssert i.uint16 + f.float64 ~= 3
+  doAssert i.uint32 + f.float ~= 3
+  doAssert i.uint32 + f.float32 ~= 3
+  doAssert i.uint32 + f.float64 ~= 3
+  doAssert i.uint64 + f.float ~= 3
+  doAssert i.uint64 + f.float32 ~= 3
+  doAssert i.uint64 + f.float64 ~= 3
+
+  doAssert f.float + i.int  ~= 3
+  doAssert f.float32 + i.int ~= 3
+  doAssert f.float64 + i.int ~= 3
+  doAssert f.float + i.int8 ~= 3
+  doAssert f.float32 + i.int8 ~= 3
+  doAssert f.float64 + i.int8 ~= 3
+  doAssert f.float + i.int16 ~= 3
+  doAssert f.float32 + i.int16 ~= 3
+  doAssert f.float64 + i.int16 ~= 3
+  doAssert f.float + i.int32 ~= 3
+  doAssert f.float32 + i.int32 ~= 3
+  doAssert f.float64 + i.int32 ~= 3
+  doAssert f.float + i.int64 ~= 3
+  doAssert f.float32 + i.int64 ~= 3
+  doAssert f.float64 + i.int64 ~= 3
+  doAssert f.float + i.uint ~= 3
+  doAssert f.float32 + i.uint ~= 3
+  doAssert f.float64 + i.uint ~= 3
+  doAssert f.float + i.uint8 ~= 3
+  doAssert f.float32 + i.uint8 ~= 3
+  doAssert f.float64 + i.uint8 ~= 3
+  doAssert f.float + i.uint16 ~= 3
+  doAssert f.float32 + i.uint16 ~= 3
+  doAssert f.float64 + i.uint16 ~= 3
+  doAssert f.float + i.uint32 ~= 3
+  doAssert f.float32 + i.uint32 ~= 3
+  doAssert f.float64 + i.uint32 ~= 3
+  doAssert f.float + i.uint64 ~= 3
+  doAssert f.float32 + i.uint64 ~= 3
+  doAssert f.float64 + i.uint64 ~= 3