N_IDENT for a slice local now loads (AX=ptr, BX=len, CX=cap), matching
the existing global-slice load.
cgexpr learns N_SLICE: `base[lo:hi]` leaves the same triple in
registers, so callers (return, arg push, reassignment) all share
one shape. The let-init's pre-existing N_SLICE direct-store path
stays as a specialisation; the new generic slice let-init catches
fn-returning-slice and slice-ident initialisers.
N_ASSIGN gains a TY_SLICE branch parallel to TY_STR: store all
three halves to the local slot or, for globals, stash CX into DI
before LEAQ-ing the address (CX is both the new cap and the
address scratch).
Hojun-Cho
548547a1d0