Compiler: keep multiple occurrences of string "number"

JavaScript engines recognize 'typeof x==="number"' and generate
efficient code for this expression. We use this pattern when compiling
pattern matching. Replacing the string by a variable can have a
significant performance impact.
The space overhead of this change is negligeable (< 0.2%)
Compiler: perform some array bound-checking optimizations
Runtime: MlString.safeGet should return unit
Makefiles: put binaries at the same place as ocamlc by default
Merge branch 'master' of https://github.com/ocsigen/js_of_ocaml