fix std.mem.addNullByte and implement sentinel slicing

[andrewrk]

Closes #3770

checklist:

• add missing compile error for type coercing non-sentinel-terminated slice to sentinel-terminated slice
• fix std.mem.addNullByte
• implement slicing syntax which can obtain sentinel terminated types from non-sentinel-terminated types #3770
• add runtime safety panic test for slicing arrays
• add runtime safety panic test for slicing pointers
• add runtime safety panic test for slicing slices
• slicing a sentinel-terminated pointer without this syntax yields a non-sentinel-terminated result.
• add runtime safety panic test when sentinel is float
• add runtime safety panic test when sentinel is optional pointer
• index of a sentinel-terminated array which is comptime-known to be len, is a comptime-known value when loading. no-op with safety check when storing. moved to improved handling of index of a sentinel-terminated array which is comptime-known to be len #3962
• add compile error for sentinel assertion failed moved to add compile error for sentinel assertion failed #3963
• zig fmt new syntax
• fix test regressions