Sequence construction API refactoring using elements function

[stepancheg]

Before this commit Vector functions were over-overloaded: call to

Vector[T](a)

could interpret a as a sequence or as an element parameter. This is over-overloading that make code harder to read and maintain.

This commit changes sequence construction API. New convention is proposed:

Let Foo be sequence type.

Calls

Foo(arg)
Foo[T](arg)

assume arg is a sequence. Call

foo(..args)

interpret args as elements for new foo sequence. Latter call does not work when args list is empty (because type parameter for Foo cannot be infered and thus must be specified).

To deal with this problem, elements function is proposed:

Foo[T](elements(..args))

elements function with non-empty parameters list returns an array of arguments, and elements with empty args returns special sequence-like record NoElements, that has no type parameter (thus elements() call does not fail, unlike vector()).

This patch changes vector, vectorQueue and deque.

Alternative version

Alternative version of this pull request: #483, that uses tuples for parameters instead of elements function.

[jckarter]

Another approach that wouldn't need a new function would be to use the keyword syntax, as in Vector[Int](elements:[1,2,3]).

[stepancheg]

@jckarter if it's OK to pass elements in a tuple, we don't need a keyword:

Vector[Int]([1, 2, 3])