Update conversion to strings on exceptions

[henriquemoody]

Update conversion to strings on exceptions

Many changes were made on ValidationException::stringify:

• Add support for instances of Exception;
• Add support for instances of Traversable;
• Add support for resources;
• Improve Array conversion;
• Improve Object conversion;
• Improve conversion of all values by using JSON.

Now, all the parameters of the exception classes are just converted to string when replacing parameters on exceptions, so the exception classes now keep the original value of all parameters.

Fix #395

---

Samples:

""
"foo"
INF
-INF
NaN
123
123.456
{ }
{ false }
{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }
{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,  ...  }
{ "foo": true, "bar": { "baz": 123, "qux": { 1, 2, 3 } } }
{ "foo": true, "bar": { "baz": 123, "qux": { "norf": ... } } }
{ { }, "foo" }
{ { 1 }, "foo" }
{ 1, { 2, { 3 } } }
{ 1, { 2, { 3, ... } } }
{ 1, { 2, { 3, ... } } }
{ "foo", "bar" }
{ "foo", -1 }
"stringify.phpt"
"1988-09-09T23:59:59+00:00"
`[object] (stdClass: { })`
`[object] (stdClass: { "foo": 1, "bar": false })`
`[object] (stdClass: { "name": [object] (stdClass: ...) })`
`[exception] (Exception: { "message": "My message", "code": 0, "file": "%s:%d" })`
`[traversable] (ArrayIterator: { 1, 2, 3 })`
`[traversable] (ArrayIterator: { "a": 1, "b": 2, "c": 3 })`
`[resource] (stream-context)`
`[resource] (stream)`
`[resource] (xml)`
{ `[object] (stdClass: { "foo": 1, "bar": false })`, { 42, 43 }, true, null, `[resource] (stream)` }