Added .build/ folder to .gitignore.
Added more scribbles.
Added plisp_tree_root() function to src/tree.h.
Added tree unit tests.
Added typedef for c function pointer type.
Removed arguments from lambda and C function plisp_node_t subtypes.

The arguments will be extracted from the runtime stack when the tree is evaluated.
Updated comments.
Added unimplemented creator functions for all plisp_node_t subtypes.
Implemented basic tree management functions. Passes written unit tests.
Removed LIBP submodule and replaced it with libs/ folder.

The libs/ folder contains smaller submodules, which makes the PLISP repository more streamlined.