bpo-41513: Improve speed and accuracy of math.hypot()

[rhettinger]

https://bugs.python.org/issue41513

[serhiy-storchaka]

If it requires to make tests less strict, it is potentially breaking change, and therefore it should be mentioned in What's New.

[serhiy-storchaka]

What is faster, x *= scale or x = ldexp(x, -max_e)?

[rhettinger]

• The test as written was over-specified. It should have been written this way from the start.

• The x *= scale is faster than x = ldexp(x, -max_e). The former is a single, fast in-line instruction and the latter is an external library call.

Here's the generated code for the loop:

L284:
    movsd   (%r12,%rax,8), %xmm0
    addq    $1, %rax
    cmpq    %rax, %rbp
    mulsd   %xmm4, %xmm0           <-- x *= scale
    movapd  %xmm0, %xmm1
    mulsd   %xmm0, %xmm1           <-- x *= x
    movapd  %xmm2, %xmm0
    addsd   %xmm1, %xmm2            <-- csum += x
    subsd   %xmm2, %xmm0
    addsd   %xmm1, %xmm0
    addsd   %xmm0, %xmm3
    jg  L284
    subsd   %xmm