put empty generic lists behind a pointer

[arielb1]

This reduces the size of hir::Expr from 128 to 88 bytes (!) and shaves
200MB out of #36799.

This is a performance-sensitive PR so please don't roll it up.

r? @eddyb

[eddyb]

@bors p=1 r+

[bors]

📌 Commit c1ca233 has been approved by eddyb

[arielb1]

problem compiling rustdoc. Very done for today. Will fix on Sunday.

[00:36:02] error[E0609]: no field types on type &std::option::Option<syntax::ptr::P<rustc::hir::PathParameters>>

@bors r-

[arielb1]

@bors r=eddyb

[bors]

📌 Commit e921b32 has been approved by eddyb

[bors]

⌛ Testing commit e921b32 with merge be962b0e0d7cd32f642cb3a304dbf9fc74e2900d...

[bors]

💔 Test failed - status-travis

[Mark-Simulacrum]

@bors retry

cc @alexcrichton -- possibly an issue with Cargo/crates.io?

[00:04:51] extracting /Users/travis/build/rust-lang/rust/build/cache/2017-08-29/rustc-beta-i686-apple-darwin.tar.gz

[00:04:52] downloading https://static.rust-lang.org/dist/2017-08-29/cargo-beta-i686-apple-darwin.tar.gz

[00:04:52] 
################################                                          44.6%
#######################################################                   76.5%
######################################################################## 100.0%

[00:04:55] extracting /Users/travis/build/rust-lang/rust/build/cache/2017-08-29/cargo-beta-i686-apple-darwin.tar.gz

[00:04:57]     Updating registry `https://github.com/rust-lang/crates.io-index`



No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

The build has been terminated

[bors]

⌛ Testing commit e921b32 with merge 48c1c54...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 48c1c54 to master...