Add IdentifierIsh alias, update identifier function

For #19.
Add ParametersIsh alias and parameters function

For #19.
Add BodyIsh alias and utility functions

Utility functions classBody, interfaceBody and block.
Add ceylon.ast.create module (for #19)

Two new modules: ceylon.ast.create and test.ceylon.ast.create. All
existing “sugaring” moves to ceylon.ast.create, and the tests (xFunction
in test.ceylon.ast.core) move to test.ceylon.ast.create.
Add create function for CaseTypes
Add test for create function for CaseTypes
Add create function for SatisfiedTypes
Add test for create function for SatisfiedTypes
Improve create identifier functions

- IdentifierIsh parameter (instead of String)
- More documentation
Improve create {Base,Qualified}{Expression,Type} functions

IdentifierIsh parameters instead of String
Add create function for PrimaryType

For use in other functions; not useful by itself.
Add test for create function for PrimaryType
Improve create CaseTypes function

PrimaryTypeIsh and IdentifierIsh instead of PrimaryType and MemberName.
Improve create SatisfiedTypes function

PrimaryTypeIsh instead of PrimaryType
Remove unnecessary import
Improve create CaseTypes, SatisfiedTypes functions

- Variadic
- Simplified

See #64; for #19.
Add create ExtendedType, ClassSpecifier functions

(And ClassInstantiation, but I might unshare that, it’s really just
auxiliary for the other two – I don’t see why you’d want to use it
directly.)

For #19.
Add tests for create ExtendedType, ClassSpecifier functions
Fix name of create positional arguments test
Add create function for TypeParameters

For #19.