With this change zig ld can link with dynamic libraries
contained within a fat/universal file that had multiple
seperate binaries embedded within it for multi-arch
support (in macOS).
Whilst zig can still only create single-architecture
executables - the ability to link with fat libraries is
useful for cases where they are the easiest (or only)
option to link against.
After giving it more thought, it doesn't make sense to separate
the two structurally. Instead, there should be two constructors
for a Dylib struct: one from binary file, and the other from a stub
file. This cleans up a lot of code and opens the way for recursive
parsing of re-exports from a dylib which are a hard requirement for
native feel when linking frameworks.
Instead of trying to fit a stub file into the frame of a Dylib struct,
I think it makes more sense to keep them as separate entities with
possibly shared interface (which would be added in the future).
This cleaned up a lot of logic in Dylib as well as Stub. Also, while
here I've made creating actual *Symbols lazy in the sense Dylib and
Stub only store hash maps of symbol names that they expose but we
defer create and referencing given dylib/stub until link time when
a symbol is actually referenced. This should reduce memory usage
and speed things up a bit.
Normally we rely on importing std to in turn import the root
in the start code, but when using the stage1 won't happen,
so in order to run AstGen on the root we put it into the
import_table here.
There is now a distinction between `@import` with a .zig extension and
without. Without a .zig extension it assumes it is a package name, and
returns error.PackageNotFound if not mapped into the package table.
This change reduces the amount of divergence in the compiler's main
pipeline logic enough to run AstGen for all files in the compilation,
regardless of whether the stage1 or stage2 backend is being used.
Practically, this means that all Zig code is subject to new compile
errors, such as unused local variables.
Additionally:
* remove leftover unsound asserts from recent hash map changes
* fix sub-Compilation errors not indenting correctly
