Tidy up tt.h

Backport some non-functional changes
found working on 'dense TT' patch.

No functional change.
Fix a warning with Intel compiler

warning #2259: non-pointer conversion from
"int" to "int16_t={short}" may lose significant
bits.

No functional change.
Revert and merge with 'master'

bench: 8321835
Pad TTEntry to 16 bytes

So that a cluster of size 4 is one
cache line of 64 bytes.

bench: 8321835