Revert the last two commits in this branch

When the slice-by-length start position is runtime-known, it is likely
protected by a runtime-known condition and therefore a compile error is
less appropriate than a runtime panic check.

This is demonstrated in the json code that was updated and then reverted
in this commit.

When ziglang#3806 is implemented, this decision can be reassessed.

Revert "std: work around compiler unable to evaluate condition at compile time"
Revert "frontend: comptime array slice-by-length OOB detection"

This reverts commit 7741aca.
This reverts commit 2583b38.

Author: andrewrk

compile_errors/out of bounds array slice by length.zig

@@ -2,14 +2,14 @@ const std = @import("std");
 
 pub fn panic(message: []const u8, stack_trace: ?*std.builtin.StackTrace, _: ?usize) noreturn {
     _ = stack_trace;
-    if (std.mem.eql(u8, message, "index out of bounds: index 9, len 5")) {
+    if (std.mem.eql(u8, message, "index out of bounds: index 16, len 5")) {
         std.process.exit(0);
     }
     std.process.exit(1);
 }
 pub fn main() !void {
     var buf: [5]u8 = undefined;
-    _ = buf[foo(6)..][0..3];
+    _ = buf[foo(6)..][0..10];
     return error.TestFailed;
 }
 fn foo(a: u32) u32 {