API Glue inside the library proper

I just moved the files over to the other directory and adjusted the Makefiles.

I am thinking these functions should come with just including `hammer.h`, but would like your input on how to integrate. Include `glue.h` from `hammer.h` or dump them all into `hammer.h` and remove `glue.h`?

PS: There are two other changes in there that cropped up while going over the base64 example (see other pull request): Fixed a bug in one function (`h_seq_append`) and added a new helper to construct sequences with a known allocation size (`H_MAKE_SEQN`).

PPS: I also suppose the glue stuff should get doxygen-commented somehow...?