Embed func_t microkernel objects in control trees.

Details:
- Modified all control tree node definitions to include a new field of
  type func_t*, which is similar to a blksz_t except that it contains
  one function pointer (each typed simply as void*) for each datatype.
  We use the func_t* to embed pointers to the micro-kernels to use for
  the leaf-level nodes of each control tree. This change is a natural
  extension of control trees and will allow more flexibility in the
  future.
- Modified all macro-kernel wrappers to obtain the micro-kernel pointers
  from the incomming (previously ignored) control tree node and then pass
  the queried pointer into the datatype-specific macro-kernel code, which
  then casts the pointer to the appropriate type (new typedefs residing
  in bli_kernel_type_defs.h) and then uses the pointer to call the micro-
  kernel. Thus, the micro-kernel function is no longer "hard-coded" (that
  is, determined when the datatype-specific macro-kernel functions are
  instantiated by the C preprocessor).
- Added macros to bli_kernel_macro_defs.h that build datatype-specific
  base names if they do not exist already, and then uses those to build
  datatype-specific micro-kernel function names. This will allow
  developers extra flexibility if they wanted to, for example, name each
  of their datatype-specific micro-kernels differently (e.g. double
  real might be named bli_dgemm_opt_4x4() while double complex might be
  named bli_zgemm_opt_2x2()).
- Inserted appropriate code into _cntl_init() functions that allocates
  and initializes a func_t object for the corresponding micro-kernels.
  The gemm ukernel func_t object is created once, in bli_gemm_cntl_init(),
  and then reused via extern wherever possible.