lib/n/builtins: Unreached: print a message before aborting
lib/n/builtins: rename Trim_{float,double} to Round_{float,double}

Although, we would still like to make clear that the rounding behavior
is ill specified in general. So maybe the name should reflect that
better.
bootstrap: rename typset to fintypset

fintypset because this structure is really only capable of handling
final typs. Tentative typs can be used but only with extreme care
(cannot be relinked while in the fintypset).
bootstrap:types: typ_isa(): enable the use of quickisa

The cost of typ_isa() was significant, not anymore.

It is important to note that there are likely simpler solution to the
cost of a recursive typ_isa():
 - (a) quickisa: precompute common answers in typset stored in each typ;
 - (b) caching: store the result of typ_isa() for quick re-use;
 - (c) special case: most of the typ_isa() calls, at least on the
   current testing code-base, are related to tests like
	(Ref U8) isa (`Any_ref `Any)
   (note the `Any on the RHS). We would likely be able to speed up
   a naive typ_isa() by simply hand-coding these cases.

A combination of options (b) and (c) would likely be enough (and maybe
simpler than quickisa). An alternative to consider for ncc.
bootstrap:lexer: fix (spurious) uninitialized var warning
1/xx: bootstrap:types: store more in 'typ': gen_arity
wip