Grow the decode arena through a function and store its position back on every return

slow_string() kept its write position in locals and grew the arena through
the macro NLOHMANN_VIEW_ENSURE. The macro is now the always-inline member
arena_ensure() (a lambda is not inlined at every call site by clang, which
then keeps the position in memory: +11 % instructions on escape-heavy
strings), and a guard stores the position back into m_arena_write on every
return instead of by hand on the successful one. Instruction counts are
unchanged (clang) or 1 % lower (GCC).

Also word the complexity of back() as that of at().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/back.md

@@ -28,7 +28,7 @@ This exception does not carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.
 
 Linear in the size of the array or object: unlike [`front()`](front.md), which only ever looks at the first
 element, `back()` has to walk every element to find where they end, since elements are not a fixed size in the
-index. Constant for an array none of whose elements is an array or object, since the elements are then adjacent
+index. Constant if no element of the array is an array or object, since the elements are then adjacent
 in the index.
 
 ## Notes