Gets a collection of rank specifiers for the array. Rank specifiers represent each bracketed section of an array ([,,][,] indicates two rank specifiers). Only the first rank specifier can hold size information, so there is no need for a collection of rank specifier sizes. Note: the .net documentation can be confusing on these terms, as it uses 'rank' and 'dimensions' interchangeably, and 'jagged arrays' for these types of arrays of arrays. This is fine, but leaves us with 'a jag' to refer to a rank specifier. The term 'RankSpecifiers' is thus derived from the C# spec instead.