rustpkg: Support sub-package-IDs

Package IDs can now refer to a subdirectory of a particular source
tree, and not just a top-level directory with a src/ directory as its
parent.

For example, referring to the package ID a/b/c/d , in workspace W,
if W/src/a is a package, will build the sources for the package in
a/b/c/d (and not other crates in W/src/a).

Closes #6408
auto merge of #9146 : catamorphism/rust/rustpkg-sub-package-ids, r=brson,catamorphism

r? @brson Package IDs can now refer to a subdirectory of a particular source
tree, and not just a top-level directory with a src/ directory as its
parent.

For example, referring to the package ID a/b/c/d , in workspace W,
if W/src/a is a package, will build the sources for the package in
a/b/c/d (and not other crates in W/src/a).

Closes #6408
rustpkg: Search RUST_PATH properly for dependencies, and add a test for recursive dependencies

Closes #8524
auto merge of #9132 : catamorphism/rust/rustpkg-recursive-deps, r=catamorphism,metajack

r? @metajack ...recursive dependencies

Closes #8524
rustpkg: Install to RUST_PATH

Install to the first directory in the RUST_PATH if the user set a
RUST_PATH. In the case where RUST_PATH isn't set, the behavior
remains unchanged.

Closes #7402
auto merge of #9147 : catamorphism/rust/rustpkg-install-to-rust-path, r=catamorphism,metajack

r? @metajack Install to the first directory in the RUST_PATH if the user set a
RUST_PATH. In the case where RUST_PATH isn't set, the behavior
remains unchanged.

Closes #7402
fix handling of function attributes

The `noalias` attributes were being set only on function definitions,
not on all declarations. This is harmless for `noalias`, but prevented
some optimization opportunities and is *not* harmless for other
attributes like `sret` with ABI implications.

Closes #9104
set sret attribute as needed on call instructions

Since function pointers do not carry along the function attributes with
them in the type, this needs to be set on the call instruction itself.

Closes #9152
add sret + noalias to the out pointer parameter

This brings Rust in line with how `clang` handles return pointers.

Example:

    pub fn bar() -> [uint, .. 8] {
        let a = [0, .. 8];
        a
    }

Before:

    ; Function Attrs: nounwind uwtable
    define void @_ZN3bar17ha4635c6f704bfa334v0.0E([8 x i64]* nocapture, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #1 {
    "function top level":
      %a = alloca [8 x i64], align 8
      %2 = bitcast [8 x i64]* %a to i8*
      call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
      %3 = bitcast [8 x i64]* %0 to i8*
      call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 64, i32 8, i1 false)
      ret void
    }

After:

    ; Function Attrs: nounwind uwtable
    define void @_ZN3bar17ha4635c6f704bfa334v0.0E([8 x i64]* noalias nocapture sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #1 {
    "function top level":
      %2 = bitcast [8 x i64]* %0 to i8*
      call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
      ret void
    }

Closes #9072
Closes #7298
remove unused `FastCall` wrapper

this can just be done using CallWithConv
teach Call/CallWithConv to set attributes