Add custom collation functions

[special]

// CreateCollation registers a Go function as a SQLite collation function.
//
// These function are used with the COLLATE operator to implement custom sorting in queries.
//
// The xCompare function must return an integer that is negative, zero, or positive if the first
// string is less than, equal to, or greater than the second, respectively. The function must
// always return the same result for the same inputs and must be commutative.
//
// These are the same properties as strings.Compare().
//
// https://sqlite.org/datatype3.html#collation
// https://sqlite.org/c3ref/create_collation.html
func (conn *Conn) CreateCollation(name string, xCompare func(string, string) int) error { 

[AdamSLevy]

Looks good. minor changes please:

• use RWMutex
• Remove conn from xcollation

[AdamSLevy]

I don't think a collation ever needs to know its own conn actually.

[AdamSLevy]

Please make this a sync.RWMutex

[AdamSLevy]

Suggested change

	xcollations.mu.Lock()
	x := xcollations.m[int(ptr)]
	xcollations.mu.Unlock()
	xcollations.mu.RLock()
	x := xcollations.m[int(ptr)]
	xcollations.mu.RUnlock()