Add enter/exit logs to napi_* calls

[bartlomieju]

We need to have some basic mechanism to easily debug when a problem with package using Node-API occurs.

To do that I propose we add simple logging mechanism to napi_sym:

deno/cli/napi/sym/lib.rs

Lines 15 to 36 in 35e5159

	pub fn napi_sym(_attr: TokenStream, item: TokenStream) -> TokenStream {
	let func = syn::parse::<syn::ItemFn>(item).expect("expected a function");
	let exports: SymbolExports =
	serde_json::from_str(NAPI_EXPORTS).expect("failed to parse exports");
	let name = &func.sig.ident;
	assert!(
	exports.symbols.contains(&name.to_string()),
	"tools/napi/sym/symbol_exports.json is out of sync!"
	);
	let block = &func.block;
	let inputs = &func.sig.inputs;
	let generics = &func.sig.generics;
	TokenStream::from(quote! {
	// SAFETY: it's an NAPI function.
	#[no_mangle]
	pub unsafe extern "C" fn #name #generics (#inputs) -> napi_status {
	#block
	}
	})
	}

We should log when the function enters:
napi_<function_name>
and then when it exits:
napi_<function_name> - <napi_status>.

It's probably worth to add a simple counter so we can more easily trace threadsafe function calls.

To enable tracing I propose we add DENO_NAPI_TRACING=true env var.