Zir: refactor declaration instruction representation

The new representation is often more compact. It is also more
straightforward to understand: for instance, `extern` is represented on
the `declaration` instruction itself rather than using a special
instruction. The same applies to `var`, making both of these far more
compact.

This commit also separates the type and value bodies of a `declaration`
instruction. This is a prerequisite for ziglang#131.

In general, `declaration` now directly encodes details of the syntax
form used, and the embedded ZIR bodies are for actual expressions. The
only exception to this is functions, where ZIR is effectively designed
as if we had ziglang#1717. `extern fn` declarations are modeled as
`extern const` with a function type, and normal `fn` definitions are
modeled as `const` with a `func{,_fancy,_inferred}` instruction. This
may change in the future, but improving on this was out of scope for
this commit.

src/InternPool.zig

@@ -2018,7 +2018,6 @@ pub const Key = union(enum) {
         ty: Index,
         init: Index,
         owner_nav: Nav.Index,
-        lib_name: OptionalNullTerminatedString,
         is_threadlocal: bool,
         is_weak_linkage: bool,
     };
@@ -2741,7 +2740,6 @@ pub const Key = union(enum) {
                 return a_info.owner_nav == b_info.owner_nav and
                     a_info.ty == b_info.ty and
                     a_info.init == b_info.init and
-                    a_info.lib_name == b_info.lib_name and
                     a_info.is_threadlocal == b_info.is_threadlocal and
                     a_info.is_weak_linkage == b_info.is_weak_linkage;
             },
@@ -5573,9 +5571,6 @@ pub const Tag = enum(u8) {
         /// May be `none`.
         init: Index,
         owner_nav: Nav.Index,
-        /// Library name if specified.
-        /// For example `extern "c" var stderrp = ...` would have 'c' as library name.
-        lib_name: OptionalNullTerminatedString,
         flags: Flags,
 
         pub const Flags = packed struct(u32) {
@@ -6928,7 +6923,6 @@ pub fn indexToKey(ip: *const InternPool, index: Index) Key {
                 .ty = extra.ty,
                 .init = extra.init,
                 .owner_nav = extra.owner_nav,
-                .lib_name = extra.lib_name,
                 .is_threadlocal = extra.flags.is_threadlocal,
                 .is_weak_linkage = extra.flags.is_weak_linkage,
             } };
@@ -7575,7 +7569,6 @@ pub fn get(ip: *InternPool, gpa: Allocator, tid: Zcu.PerThread.Id, key: Key) All
                     .ty = variable.ty,
                     .init = variable.init,
                     .owner_nav = variable.owner_nav,
-                    .lib_name = variable.lib_name,
                     .flags = .{
                         .is_const = false,
                         .is_threadlocal = variable.is_threadlocal,