splat: Rename broadcast() to splat##() and broadcast_w() to splat()

Implicit lane width is not only confusing, but also not future proof
lest we will want to add 256-bit wide splat() function.

This is API-breaking change.
transpose: Remove transpose16()

This function is very unlikely to be used in practice

This is API-breaking change.
Rename zip_lo() to zip##_lo() and zip_hi() to zip##_hi()

Implicit lane width is not only confusing, but also not future proof
lest we will want to add truly 256-bit wide zip_hi() or zip_lo()
functions.

This is API-breaking change.
insert: Fix static assertion in vec_insert()

Also, if the base vector lengths are not same, combining or splitting is
needed
insert: Fix a silly bug in vec_insert()
insert: Fix a bug in v_combine()