cpp/string/basic string/reserve

@1@ Informs a object of a planned change in size, so that it can manage the storage allocation appropriately.
 * If is greater than the current capacity, new storage is allocated, and capacity is made equal or greater than.

@@ If a capacity change takes place, all iterators and references, including the past-the-end iterator, are invalidated.

@2@ A non-binding shrink-to-fit request. After this call, capacity has an unspecified value greater than or equal to size.

Return value
(none)

Exceptions
Throws std if is greater than max_size.

May throw any exceptions thrown by, such as std.

Complexity
At most linear in the size of the string.