Add ExternalPageResource and allow discontiguous VM space

[qinsoon]

This PR changes VMSpace to multiple discontiguous regions as VM space. This PR is used in mmtk/mmtk-julia#71 to support system and package images.

• add an ExternalPageResource to manage the discontiguous regions.
• implement VMSpace with ExternalPageResource. VMSpace no longer uses ImmortalSpace.
• rename lazy_init_vm_space to set_vm_space, as we allow setting vm space multiple times with non overlapping regions.

[qinsoon]

Currently we cannot run with MSRV 1.61 at the moment. We have a dependency chain mmtk->env_logger->is_terminal, is_terminal updated to MSRV 1.63 in 0.4.8 and env_logger uses is_terminal 0.4.0 (including any point versions). We can wait until they resolve the issue. Otherwise, we can bump our MSRV. rust-cli/env_logger#273

[qinsoon]

From rust-cli/env_logger#273, the options for us are:

1. Bump our MSRV to 1.63 -- this will resolve the issue for now, but we may see the same problem in the future until cargo/Rust improves on version resolution for MSRV.
2. Check in our Cargo.lock and specify a version that is compatible with MSRV 1.61. This is not recommended by the current Rust documentation, but is in use by some Rust official projects (like env_logger) and there is an issue that suggest to make it recommended to check in lock file for libraries Check Cargo.lock in version control for libraries rust-lang/cargo#8728.

I would suggest 2. It does not look like cargo/Rust will improve on this any time soon, and 2 does no harm.

[wks]

We can also play our old trick of locking a dependency to a specific version until the problem is solved. We once did this for enum_map.

diff --git a/Cargo.toml b/Cargo.toml
index db4da419d..6f07eb851 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -28,6 +28,7 @@ delegate = "0.9.0"
 downcast-rs = "1.1.1"
 enum-map = "2.4.2"
 env_logger = "0.10.0"
+is-terminal = "=0.4.7"  # latest version requires MSRV 1.63
 itertools = "0.10.5"
 jemalloc-sys = { version = "0.5.3", features = ["disable_initial_exec_tls"], optional = true }
 lazy_static = "1.1"

[qinsoon]

binding-refs
JULIA_BINDING_REPO=qinsoon/mmtk-julia
JULIA_BINDING_REF=update-julia-upstream-43bf2c8

[qinsoon]

This PR is ready for review again. @wks

[wks]

LGTM