jets: manual noun splitting instead of u3r_mean

[dozreg-toplud]

@joemfb I tried replacing u3r_mean calls with explicit core splitting. Some observations:

1. Compiler did eliminate redundant deconstructions;
2. +ride call on Hoon source seems ~5% slower with explicit splitting.

The way I implemented it was:

1. Define macros that take u3_noun or u3_weak and return u3_weak:

#define u3r_head_weak(SOM) ({                   \
  u3_weak _som = SOM;                           \
  ( u3_none == _som ) ? u3_none                 \
    : (_(u3ud(_som))) ? u3_none                 \
    : ( ((u3a_cell *)u3a_to_ptr(_som))->hed );  \
})

#define u3r_tail_weak(SOM) ({                   \
  u3_weak _som = SOM;                           \
  ( u3_none == _som ) ? u3_none                 \
    : (_(u3ud(_som))) ? u3_none                 \
    : ( ((u3a_cell *)u3a_to_ptr(_som))->tel );  \
})

#define u3r_head(SOM) ({                      \
  u3_noun _som = SOM;                         \
  ( _(u3ud(_som)) ) ? u3_none                 \
  : ( ((u3a_cell *)u3a_to_ptr(_som))->hed );  \
})

#define u3r_tail(SOM) ({                      \
  u3_noun _som = SOM;                         \
  ( _(u3ud(_som)) ) ? u3_none                 \
  : ( ((u3a_cell *)u3a_to_ptr(_som))->tel );  \
})

2. Apply a generated python script to all files in /jets directory, turning u3r_mean into composition of the function-like macros defined above

Am I missing some way to make it more efficient?

[dozreg-toplud]

Microbenchmarking showed order of magnitude improvement though