Allow to typehint for the type (array/hash) of the root item to be serialized

[goetas]

BC version of #698

Fixes schmittjoh/JMSSerializerBundle#373 #709 #706 schmittjoh/JMSSerializerBundle#375

Closes #475 #248 #234

Examples:

// default / current (let the PHP's json_encode function decide (and sometimes the serializer :/))
$serializer->serialize([1, 2]); //  [1, 2]
$serializer->serialize(['a', 'b']); //  ['a', 'b']
$serializer->serialize(['c' => 'd']); //  {"c" => "d"}
$serializer->serialize([1, 2], SerializationContext::create()->setInitialType('array')); //  [1, 2]
$serializer->serialize([1 => 2], SerializationContext::create()->setInitialType('array')); //  {"1": 2}
$serializer->serialize(['a', 'b'], SerializationContext::create()->setInitialType('array')); //  ['a', 'b']
$serializer->serialize(['c' => 'd'], SerializationContext::create()->setInitialType('array')); //  {"c" => "d"}

// typehint as strict array, keys will be always discarded 
$serializer->serialize([], SerializationContext::create()->setInitialType('array<integer>')); //  []
$serializer->serialize([1, 2], SerializationContext::create()->setInitialType('array<integer>')); //  [1, 2]
$serializer->serialize(['a', 'b'], SerializationContext::create()->setInitialType('array<integer>')); //  ['a', 'b']
$serializer->serialize(['c' => 'd'], SerializationContext::create()->setInitialType('array<string>')); //  ["d"]

// typehint as hash, keys will be always considered
$serializer->serialize([], SerializationContext::create()->setInitialType('array<integer,integer>')); //  {}
$serializer->serialize([1, 2], SerializationContext::create()->setInitialType('array<integer,integer>')); //  {"0" : 1, "1" : 2}
$serializer->serialize(['a', 'b'], SerializationContext::create()->setInitialType('array<integer,integer>')); //  {"0" : "a", "1" : "b"}
$serializer->serialize(['c' => 'd'], SerializationContext::create()->setInitialType('array<string,string>')); //  {"d" : "d"}

[goetas]

@schmittjoh what do you think?

[goetas]

After thinking for some time on how to deal with this, i can suggest:

1. modify existing syntax/behaviour

array # let the json_encode function decide
array<foo> # always array
array<string,foo> # always hash
array<integer,foo> # always hash

The approach uses "string" and "integer", "float"... (an explicit key type definition) to define when an array is a hash.
In reality the key type definition is just ignored, since in hashes keys are always strings.

2. introduce a special parameter map or list.

array # working as before
array<foo> # working as before
array<map,foo> # always hash
array<list,foo> # always array

This approach makes it a bit differently, using the first parameter to specify when an array is a hash.

3. introduce a special type map or list.

map<foo> # always hash
list<foo> # always array

This approach is even more explicit, but introduces new types.

The approach 1 and 2 is compatible with ArrayCollection too, since is possible to define

ArrayCollection<map, User>
or 
ArrayCollection<int, User>

Currently the approach I prefer is the 2.