math/big: add SetUint64 and Uint64 functions to *Int

[agl]

(for discussion)

math/big.*Int has SetInt64 and Int64() functions for setting the value of a *Int and
getting its value respectively.

However, when interfacing with other code, it would be very useful to have similar
functions for uint64's. SetBits() works on 64-bit systems with unsafe, but that's not a
very nice solution.

[cznic]

Comment 1:

External implementation example:
https://github.com/cznic/mathutil/blob/master/mathutil.go#L292 and #L302. It could get
simpler and faster when implemented inside "math/big".

[rsc]

Comment 2:

Labels changed: added size-m.

[rsc]

Comment 3:

Labels changed: added suggested.

[Luit]

Comment 4:

I expect tests will be required? I've got an implementation. Should I put it up for
codereview, or get started on a test case or two first?
If so, what kind of tests would be necessary?

[agl]

Comment 5:

Tests would be needed, but they need only be sanity checks that, say, setting and
getting a value results in the same value and that SetUint64() followed by String()
produces the correct result.

[Luit]

Comment 6:

I created CL 6929048 with an implementation and basic tests. Let me know if there's
anything that needs changing! https://golang.org/cl/6929048

[rsc]

Comment 7:

This issue was closed by revision f4fc163.

Status changed to Fixed.