Accept RUST_CXX_NO_EXCEPTIONS to disable throwing

[dtolnay]

Partially addresses #92 (it remains to figure out how to nicely expose constructing a rust::String from a std::string when exceptions are off).

[dtolnay]

Tested by adding .flag("-fno-exceptions") in build.rs, confirming that it fails to compile, then adding .define("RUST_CXX_NO_EXCEPTIONS", None) and confirming that it then compiles.

[bruxisma]

@dtolnay might be a better choice to use std::terminate over abort, as someone can then replace the terminate handler, and possibly get a stack trace via libunwind or some other means before abort is immediately called.

[dtolnay]

Fixed in #112 to use std::terminate. Good call!