nf -> RR conversion should not be slower than nf -> RIF

[vdelecroix]

Given a quadratic number field, conversion to MPFR floating point is terribly slow compared to the more complicated MPFI interval floating point conversion

sage: %timeit RR(a)
1000 loops, best of 3: 224 µs per loop
sage: %timeit RIF(a)
1000000 loops, best of 3: 919 ns per loop

• same purpose ticket for Python floating point #26927
• related old ticket: #21991

[vdelecroix]

Implementing _mpfr_ is a complete failure (for now)...

---

New commits:

​9faa256	26839: fix evaluation of lazy constants
​c4cde9e	26839: get rid of _is_special
​1cc3774	_mpfr_ method on element of quad nf

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​ab8a836

_mpfr_ method on element of quad nf

[vdelecroix]

Declaring a _mpfr_ method is not enough to get a direct coercion path from K -> RR... see discussion at ​this sage-devel thread.

[embray]

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[embray]

Ticket retargeted after milestone closed (if you don't believe this ticket is appropriate for the Sage 8.8 release please retarget manually)

[embray]

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