using a better freeze heuristics, and fix code for memmap crate (their API changed)

See merge request !54

involved in moves (taking loop depth into consideration)

freeze heuristics, slight change to tree gen

See merge request !51

will record def of each SSA var. Instruction of int<1> will always be
put as a child (facilitate pattern matching)

merge with develop

See merge request !50

nodes

add a tool 'trace-filter' to help viewing Zebu trace

See merge request !49

Tree gen fix

See merge request !48

Develop

See merge request !47

multiple fixes and improvements

See merge request !46

emit shift instead of mul/div

frozen moves (prioritize more frequent moves, determined by node
weight/spillcost)

put binop/move as tree child

Ir formatting

See merge request !45

(it is theoratically possible that we cannot find a node to spill)