Let keep_fnames mangle implicit function names (variable names of arrow functions)

[skerit]

With keep_fnames enabled, Terser refuses to mangle the variable of the following code:

const my_long_variable_name = () => {};

I can see the reasoning behind this, but I only want to keep explicit function names, not implicit ones like this.

[fabiosantoscode]

This is a pretty reasonable request. Long names will increase file size. I think keep_fnames="explicit" would be a good option.