implement Float64() float64

[holiman]

Ref: #123

This PR implements

// Float64 returns the float64 value nearest to x.
//
// Note: The `big.Float` version of `Float64` also returns an 'Accuracy', indicating
// whether the value was too small or too large to be represented by a
// `float64`. However, the `uint256` type is unable to represent values
// out of scope (|x| < math.SmallestNonzeroFloat64 or |x| > math.MaxFloat64),
// therefore this method does not return any accuracy.
//
// OBS! Currently, this method is not optimized: but uses big.Int and big.Float.
func (z *Int) Float64() float64

Implementing Float64 makes it possible to use e.g. the logarithm functions in the math package.

Todo:

• Implement native to-float support without using package big

[codecov]

Codecov Report

Merging #132 (fd5dfa8) into master (29b48c8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #132   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines         1607      1615    +8     
=========================================
+ Hits          1607      1615    +8     

[holiman]

goos: linux
goarch: amd64
pkg: github.com/holiman/uint256
cpu: 12th Gen Intel(R) Core(TM) i7-1270P
BenchmarkFloat64/Float64/uint256-8                246834              4681 ns/op               0 B/op          0 allocs/op
BenchmarkFloat64/Float64/uint256-8                564375              4651 ns/op               0 B/op          0 allocs/op
BenchmarkFloat64/Float64/uint256-8                256735              4545 ns/op               0 B/op          0 allocs/op
BenchmarkFloat64/Float64/uint256-8                567476              2117 ns/op               0 B/op          0 allocs/op
BenchmarkFloat64/Float64/uint256-8                562767              2114 ns/op               0 B/op          0 allocs/op
BenchmarkFloat64/Float64/big-8                     23724             55516 ns/op           23424 B/op        510 allocs/op
BenchmarkFloat64/Float64/big-8                     18283             64730 ns/op           23424 B/op        510 allocs/op
BenchmarkFloat64/Float64/big-8                     18610             60523 ns/op           23424 B/op        510 allocs/op
BenchmarkFloat64/Float64/big-8                     19528             62700 ns/op           23424 B/op        510 allocs/op
BenchmarkFloat64/Float64/big-8                     19828             56352 ns/op           23424 B/op        510 allocs/op