Include math.Floor and math.Ceil in builtin library.

[kwshi]

In #727, I suggested pouring all of Go's math library into a builtin library. It was suggested, instead, that these math functions be implemented directly in Elvish. This is a plausible idea, and I have started exploring/attempting that. However, the floating-point floor and ceiling functions (and more generally, floating point modulo) I think cannot be trivially/efficiently implemented in Elvish, because they rely on the underlying bitwise encoding of double-precision floating points.

Thus I think it would be nice to do one (or both) of two things:

• Include floor and ceil (and maybe round, if we're feeling excited) in the builtin library.
• Include floating-point modulo, i.e. math.Modf, as a builtin arithmetic operator. I might propose that this operator be name %f, and be used like %f 8.3 3 (outputs 2.3).
• Include "truncated" (floored) division, a la Python. I propose this operator be named //, just as in Python. Then it would be used as // 8.3 3 (outputs 2).

The last suggestion is especially appealing to me, because it's, I think, the most minimal solution, and and everything else can be implemented using it.

[kwshi]

Actually, better yet, I propose that the builtin modulo be replaced by a floating-point modulo. But I also still kinda like the idea of a // operator.

[xiaq]

I agree. Let's put floor, ceil and round in a new math module, teach % how to deal with floats and add //.

[krader1961]

A math: namespace was introduced by commit 2d8a045. Also, my PR #937 introduces the floor and ceil commands as part of resolving issue #727. So it would appear this is no longer an unresolved issue.

[krader1961]

@xiaq, This can be closed now that PR #937 has been merged.