Add type 'scalar'

[sun]

Resolves #20 — The trumping argument:

	PHP function	Alias	PHPDoc type(s)
✓	is_array()		array
✓	is_bool()		bool, boolean
✓	is_callable()		callable
✓	is_double()	is_float()	double
✓	is_float()		float
✓	is_int()		int, integer
✓	is_integer()	is_int()	int, integer
✓	is_long()	is_int()	int, integer
✓	is_null()		null
✓	is_object()		object
✓	is_real()	is_float()	float
✓	is_resource()		resource
✗	is_scalar()		—
✓	is_string()		string
✓	—		mixed

(Original analysis by @ThomasWeinert.)

A poll/vote on this proposal (#20 (comment)) resulted in 46% support for it; i.e., roughly 50/50.

Since (1) there are legit use-cases, (2) this is a super-trivial addition to the spec, and (3) no one is harmed by including it (you don't have to use it), let's stop to debate such a minor thing, simply include it, and move on to more complex topics to get PSR-5 done, please.

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Note: is_numeric() is intentionally ignored and not the the topic of this PR, because it validates the content, not the type. All discussion on is_numeric() belongs into #42, not here.

[mvriel]

This change is still being discussed in #20

[mvriel]

Rejected this improvement, see #20 for more information