make additional header file to expose internal functions

[jay-sridharan]

This pull request adds an additional header file exposing APIs that are internal to tinyspline. There is a compilation warning in the header to warn the user that these methods may be unstable and change with versions, so that they are not depended on by mistake.

It additionally changed the default visibility of symbols to make these methods accessible, and adds variables to expose version information so that the library version can be checked dynamically.