Add incremental build

Since most of our builds are going to be incremental while we are developing the API, being able to iterate quickly is very important. Additionally, putting EMSCRIPTEN_BINDINGS() {} blocks in .cpp files is logically more consistent and forces each source file to include the relevant dependencies.

Old build system times

full build => 63 seconds
change src/pke/CryptoContext_em.cpp => 61 seconds

New build system times:

full build => 67 seconds
change src/pke/CryptoContext_em.cpp => 13 seconds