ggml : allocate graphs in a context

[slaren]

• Adds ggml_new_object to allocate objects in a context (internal to ggml)
• Adds ggml_new_graph, ggml_build_forward_ctx, ggml_graph_overhead to support allocating graphs in a context
• Modifies ggml_graph_compute_with_ctx to use ggml_new_object instead of a tensor
• Modifies llama.cpp to allocate the eval graph on the compute context

Fixes ggml-org/ggml#299