better beta reduction

[bobzhang]

when calculate Lambda.free_variables also return the occurence, suppose parameter expression has no side effect, if it is 0, don't do anything, if it is 1 (common cases for small function), do the substitution directly
also check:

1. analyze the purity of the expression
2. avoid global module

[bobzhang]

TODO: check when beta reduction and substitution, Alias will goto StrictOpt
more conservation only Llet (Alias , id, Lvar b) makes sense