Merge pull request #1845 from YosysHQ/eddie/kernel_speedup

kernel: speedup by using more pass-by-const-ref

diff --cc frontends/ast/ast.cc
Simple merge

diff --cc kernel/modtools.h
Simple merge

diff --cc kernel/rtlil.cc
Simple merge

diff --cc kernel/rtlil.h
Simple merge

diff --cc passes/memory/memory_share.cc
Simple merge

diff --cc passes/opt/opt_expr.cc
Simple merge

diff --cc passes/opt/share.cc
Simple merge

diff --cc passes/techmap/techmap.cc
Simple merge