New feature topic: built-in function mapping and EF.Functions

[divega]

Originally from #447.

Should cover EF.Functions.Like as well.

[bricelam]

It's over-simplified, but here's a good concept map for SQLite.

.NET	SQLite
Array.Length	length
DateTime.AddDays	datetime
DateTime.AddHours	datetime
DateTime.AddMilliseconds	strftime
DateTime.AddMinutes	datetime
DateTime.AddMonths	datetime
DateTime.AddSeconds	datetime
DateTime.AddTicks	strftime
DateTime.AddYears	datetime
DateTime.Date	date
DateTime.Day	strftime
DateTime.DayOfWeek	strftime
DateTime.DayOfYear	strftime
DateTime.Hour	strftime
DateTime.Milisecond	strftime
DateTime.Minute	strftime
DateTime.Month	strftime
DateTime.Now	datetime
DateTime.Second	strftime
DateTime.Ticks	strftime
DateTime.TimeOfDay	time
DateTime.Today	datetime
DateTime.UtcNow	datetime
DateTime.Year	strftime
Math.Abs	abs
Math.Max	max
Math.Min	min
Math.Round	round
string.Contains	instr
string.IndexOf	instr
string.Length	length
string.Replace	replace
string.Substring	substr
string.ToLower	lower
string.ToUpper	upper
string.Trim	trim
string.TrimEnd	rtrim
string.TrimStart	ltrim