Add re-entrancy support / multiple instances

[MightyPork]

This PR adds re-entrancy and support for creating multiple instances.
The only limitation is that the instances must have the same TF_Config, i.e. the same field sizes and checksum type. This shouldn't normally be a problem.

TF_Init() now allocates the instance structure. Use TF_InitStatic() if you want to allocate it manually.
The tf struct has two user fields, userdata and usertag (void* and uint32_t, respectively). Set those to whatever you like after calling TF_Init() - those can then be checked in any listener or callback (e.g. TF_ClaimTx() or TF_WriteImpl() to distinguish the instances. .userdata can be a pointer to some custom user structure.

All demos have been updated to work with this new version.