add SetBitsetFrom() to resolve #85

[SignorMercurio]

This should close #85 .

Test results:

$ go test -cover
PASS
coverage: 89.7% of statements
ok      github.com/bits-and-blooms/bitset       0.121s

[lemire]

How does that compare with the existing function...

// FromWithLength constructs from an array of integers and length.
func FromWithLength(len uint, set []uint64) *BitSet {
	return &BitSet{len, set}
}

[SignorMercurio]

How does that compare with the existing function...

// FromWithLength constructs from an array of integers and length.
func FromWithLength(len uint, set []uint64) *BitSet {
	return &BitSet{len, set}
}

According to #85, the newly added SetBitsetFrom() does not create a new BitSet instance, thus does not allocate new space. It only modifies the existing BitSet instance.

[lemire]

Merged.