slicing with comptime start and end indexes results in pointer-to-array

[andrewrk]

This implements #863. Big checklist before this can be merged.

Unify slices and single-item-pointer-to-array

• make slices support @ptrCast
• add .ptr property to single-pointer-to-array which decays it into [*] add .ptr property to single-pointer-to-array which decays it into [*] #4765
• peer type resolution for slice and single-pointer-to-array (example: []u8 and *const [2:0]u8 should be []const u8) peer type resolution for slice and single-pointer-to-array #4766
• peer type resolution for optional slice and single-pointer-to-array (example: lib/std/net.zig:819:60: error: incompatible types: '*[512]u8' and '?[]u8') peer type resolution for optional slice and single-pointer-to-array #4767

Add Behavioral Tests

• @ptrCast slice to pointer
• slice array
• slice array, array is 0 bits
• slice array, sentinel
• slice array, array is 0 len, sentinel
• slice pointer
• slice pointer, pointer is 0 bits
• slice pointer, sentinel
• slice slice
• comptime slice array
• comptime slice array, elem is 0 bits
• comptime slice array, elem is 0 bits, sentinel
• comptime slice pointer
• comptime slice pointer, elem is 0 bits
• comptime slice pointer, elem is 0 bits, sentinel
• comptime slice slice
• comptime slice slice, elem is 0 bits
• comptime slice slice, elem is 0 bits, sentinel
• slice array, preserve alignment
• slice pointer, preserve alignment
• slice slice, preserve alignment
• slice with comptime start and end and a sentinel

Follow-up Items

• grep for "863" and do all the fixups that are now possible
• var hack: usize = 0; // TODO remove this hack
• fix regressed compile error test "reading past end of pointer casted array"