Allow String.join to take {Object*} #219

Changes the parameter of String.join from {String*} strings to
{Object*} objects.

Closes #219.
Add tests for #219 (String.join takes objects)

Mostly copied from the existing String.join tests, wrapping Iterables /
Sequences around the strings.
Add type argument to Correspondence.keys

Changes Correspondence.keys from a bland Category to a Category<Key>.
Fix parseFloat trailing digits after _ bug

parseFloat would consider 1.000_00 an invalid float literal because
there are less than three digits behind the last underscore.

Also removes a test that tested this exact behavior.
Add tests for parseFloat trailing digits after _ bug
minor improvements to sequential()
clarification
one more clarification
remove sequential() function
fix bug in accidentally-committed code
sync Ceylon and Java definitions of Array
Array.java: fixed annotations on getSequence now that it's refined on Array.ceylon
LazyIterable: override empty/size where possible #1661
Metamodel: allow access to ModuleManager for runtime model loader tests ceylon/ceylon-compiler#1642
Merge pull request #464 from lucaswerkmeister/master

Add type argument to Correspondence.keys
Util: Add some Javadoc.
Try to remove use of SequenceBuilder from Util.sequentialInstance.
Implement Iterable.sequence as [*this].
Fix a the comprehensions test (for the JVM at least, see ceylon/ceylon-compiler#1660).
Issue #1661: Override longerThan() and shorterThan() too.