Implements a dynamic string. This string can be appended to and otherwise used as a String.

Factory Messages
create Creates a new Dynamic String
create: aString Creates a new Dynamic String initialised with aString

Operational Messages
add: aString self Appends anObject to the receiver. Answers anObject.
cr self Appends a newline sequence to the receiver.
tab self Appends a tab character to the receiver.
asString String Returns the receiver as a String