add more dynamic-length methods to tablegen

[frolic]

Using table libs, I want to be able to:

• set at index (we have "get at index" but not corresponding set)
• slice, i.e. get a subset of dynamic data (subset of items of array field or slice of bytes/string field)
• splice, i.e. replace a subset of dynamic data (replace items at position in array field, push items in array field, replace part of string, append to string)