Add front methods to json_pointers (implements #4889) (#5152)

* Added front, pop_front, and push_front methods to json_pointers in order to facilitate root-to-leaf traversals of JSON object trees. (#4889)

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* undid VS autoformatting in irrelevant code

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* Ran make amalgamate, added navigation to json_pointer's new front methods in mkdocs, and fixed errors in documented complexity for those methods.

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* Fixed GCC 4.8 compile error caused by const iterators

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* Fixed another gcc-4.8 compile error

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

* amalgamated

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

---------

Signed-off-by: trdesilva <5818730+trdesilva@users.noreply.github.com>

docs/mkdocs/mkdocs.yml

@@ -233,6 +233,7 @@ nav:
           - '(Constructor)': api/json_pointer/json_pointer.md
           - 'back': api/json_pointer/back.md
           - 'empty': api/json_pointer/empty.md
+          - 'front': api/json_pointer/front.md
           - 'operator string_t': api/json_pointer/operator_string_t.md
           - 'operator==': api/json_pointer/operator_eq.md
           - 'operator!=': api/json_pointer/operator_ne.md
@@ -240,7 +241,9 @@ nav:
           - 'operator/=': api/json_pointer/operator_slasheq.md
           - 'parent_pointer': api/json_pointer/parent_pointer.md
           - 'pop_back': api/json_pointer/pop_back.md
+          - 'pop_front': api/json_pointer/pop_front.md
           - 'push_back': api/json_pointer/push_back.md
+          - 'push_front': api/json_pointer/push_front.md
           - 'string_t': api/json_pointer/string_t.md
           - 'to_string': api/json_pointer/to_string.md
       - json_sax: