Add optional length to functions accepting const char * #254

As discussed in the last meeting, zero-terminated const char*, while popular in C land, might be problematic for consumers in other languages having different string representation (for example, Rust with non-zero-terminated string slices represented as data + length).

Some N-API functions already accept optional length that, when set to -1, would mean a length calculated from zero-terminated string and explicit byte length otherwise.

It's worth to go through a list of functions that accept const char* and add length parameter where it's missing, especially for functions that don't provide variants with napi_value as a string, namely:

• napi_create_function
• napi_define_class
• napi_module_register -> napi_module struct