C-API: avoid global variables

[HeyImJordi]

Currently the C API uses a number of global variables to store its state. This makes it hard to use e.g. in multi-threaded applications, or in libraries (think e.g. a library that loads and queries CRG files, which is then used in an application to compare multiple CRG files).

The C API should be changed to not use any mutable global variables.

[tbleher]

I think this is very desirable (and avoiding global variables would help with thread safety - see #21), but from what I understand this would be an API break - existing applications would not work without source code adaptations. So at the very least 1.3 material.