ggml: add names to tensors

[slaren]

• Adds a name field to ggml_tensor
• Adds accessor functions ggml_get_name and ggml_set_name
  • While we normally don't use accessors, in this case it seemed necessary as the maximum length may change
• Modifies ggml_graph_dump_dot to use the names in the graph
• Modifies llama.cpp to add names to most tensors

Example graph (heavily inspired by #915 (comment)):

[ggerganov]

Let's keep it to remind us