Improve some sparse arrays

[AlexanderKot]

Some sparse arrays exists after my app initialized related to NHib.

1

Object type	Wasted	Wasted After	Count	Count After
System.Collections.Concurrent.ConcurrentDictionary+VolatileNode<System.Tuple<System.Int32, System.String>, System.String[][]>[]	2,27 MB	0	8047	0

Improved by making Loader._subclassEntityAliasesMap lazy initialized
simple entities more frequent in data model then inheritance hierarchy

2

Object type	Wasted	Wasted After	Count	Count After
NHibernate.SqlCommand.SqlString+Part[]	736,47 KB	17.48 KB	20339	20343

Object type	Wasted	Wasted After	Count	Count After
NHibernate.SqlCommand.Parameter[]	410,4 KB	23.58 KB	20329	20331

Improved by TrimExcess in internal SqlString(IEnumerable parts) constructor

3

Object type	Wasted	Wasted After	Count	Count After
NHibernate.Mapping.ISelectable[]	440,4 KB	9.07 KB	18806	19966

Improved by SimpleValue.columns 1 default list size (Most simple values mapped to 1 column in DB)

Object type	Wasted	Wasted After	Count	Count After
NHibernate.Mapping.Column[]	332,78 KB	1.4 KB	14658	14658

Improved by
Constraint.columns 1 default list size (Most FKs is 1 column)
Index.columns 1 default list size (indexes created for FK usually created by 1 column)
ForeignKey.referencedColumns - make it lazy initialized as not used when referencing PK
ForeignKeyKey - TrimExcess on internal collections