[write-fonts] Table `Head` should have a function `set_loca_format`

[jqwv]

When I create the Glyf and Loca tables using

let (table_glyf, table_loca, loca_format) = glyf_loca_builder.build();

the docs say This method also returns the loca format; the caller is responsible for setting this field in the head table.

Ok. But how? The returned type is different from Head.index_to_loc_format.
Currently, I'm doing the following:

table_head.index_to_loc_format = loca_format as _;

As a normal caller I can only guess that this is

• the correct field
• the correct type conversion

Adding a function set_loca_format and then calling table_head.set_loca_format(loca_format) would make it clear.