Add coalescing assignment operators

[dnwpark]

Given a string property:

Insert with :=? operator does the following:

default	compexpr	assigned
'foo'	bar	bar
'foo'	{}	foo
{}	bar	bar
{}	{}	{}
no default	bar	bar
no default	{}	{}

Update with :=? operator does the following:

existing	compexpr	assigned
'foo'	bar	bar
'foo'	{}	foo
{}	bar	bar
{}	{}	{}

Using either operator with insert produces the following error:
coalescing assignments are prohibited outside of insert and update

Todo:

• fix null set requiring type cast
• type checking flags adding ctx to SelectQuery