Make crate compile and pass tests on aarch64 platform

**Description**
Use conditional compilation throughout the crate to make it compile and
run on aarch64 platform.

Mainly removed use of perfcnt crate and associated performance counters
on aarch64, revert to wall time for benchmark measurement.

One of the tagged pointer test started failing because u128 has a
different alignment on aarch64 versus x86. There is a rust-lang issue:
rust-lang/rust#54341

**Motivation**
This change is needed so I can develop this crate on an M1 mac.

**Testing Done**
 - `cargo test`
 - `cargo build --all-targets`
 - `cargo miri test`

Cargo.toml

@@ -11,11 +11,13 @@ features = ["std", "rustc_1_57"]
 [dev-dependencies]
 argh = "0.1.7"
 criterion = { version = "0.3.5", features = ["html_reports"] }
-criterion-perf-events = "0.1.4"
-perfcnt = "0.7.2"
 dhat = "0.3.0"
 rustc-hash = "1.1.0"
 
+[target.'cfg(target_arch = "x86")'.dependencies]
+criterion-perf-events = "0.1.4"
+perfcnt = "0.7.2"
+
 [[bench]]
 name = "raw_apis"
 harness = false

benches/raw_apis.rs

@@ -5,13 +5,22 @@ use blart::{
     },
     LeafNode, NodePtr, OpaqueNodePtr,
 };
+#[cfg(not(target_arch = "x86"))]
+use criterion::measurement::WallTime;
 use criterion::{criterion_group, criterion_main, BenchmarkGroup, Criterion};
+#[cfg(target_arch = "x86")]
 use criterion_perf_events::Perf;
+#[cfg(target_arch = "x86")]
 use perfcnt::linux::{HardwareEventType, PerfCounterBuilderLinux};
 use std::time::Duration;
 
+#[cfg(target_arch = "x86")]
+type Measurement = Perf;
+#[cfg(not(target_arch = "x86"))]
+type Measurement = WallTime;
+
 fn run_benchmarks(
-    group: &mut BenchmarkGroup<Perf>,
+    group: &mut BenchmarkGroup<Measurement>,
     key_vec: &[Box<[u8]>],
     tree_root: OpaqueNodePtr<usize>,
 ) {
@@ -44,7 +53,7 @@ fn run_benchmarks(
 }
 
 fn setup_tree_run_benches_cleanup(
-    c: &mut Criterion<Perf>,
+    c: &mut Criterion<Measurement>,
     keys: impl Iterator<Item = Box<[u8]>>,
     group_name: &str,
 ) {
@@ -64,7 +73,7 @@ fn setup_tree_run_benches_cleanup(
     unsafe { deallocate_tree(root) };
 }
 
-pub fn raw_api_benches(c: &mut Criterion<Perf>) {
+pub fn raw_api_benches(c: &mut Criterion<Measurement>) {
     // number of keys = 256
     setup_tree_run_benches_cleanup(c, generate_keys_skewed(u8::MAX as usize), "skewed");
     // number of keys = 256
@@ -89,7 +98,8 @@ pub fn raw_api_benches(c: &mut Criterion<Perf>) {
     )
 }
 
-fn create_criterion_configuration() -> Criterion<Perf> {
+#[cfg(target_arch = "x86")]
+fn create_criterion_configuration() -> Criterion<Measurement> {
     Criterion::default()
         .with_measurement(Perf::new(PerfCounterBuilderLinux::from_hardware_event(
             // I switched to using retired instruction counts because the variability of the wall
@@ -110,6 +120,13 @@ fn create_criterion_configuration() -> Criterion<Perf> {
         .measurement_time(Duration::new(10, 0))
 }
 
+#[cfg(not(target_arch = "x86"))]
+fn create_criterion_configuration() -> Criterion<Measurement> {
+    Criterion::default()
+        .sample_size(1000)
+        .measurement_time(Duration::new(10, 0))
+}
+
 criterion_group! {
     name = benches;
     config = create_criterion_configuration();

src/tagged_pointer.rs

@@ -423,12 +423,25 @@ mod tests {
             0b11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111000usize
         );
 
-        assert_eq!(TaggedPointer::<u128>::ALIGNMENT, 8);
-        assert_eq!(TaggedPointer::<u128>::NUM_BITS, 3);
-        assert_eq!(
-            TaggedPointer::<u128>::POINTER_MASK,
-            0b11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111000usize
-        );
+        // Something weird about the representation of u128 on x86 vs aarch64 platforms: 
+        // https://github.com/rust-lang/rust/issues/54341
+        if cfg!(target_arch = "x86") {
+            assert_eq!(TaggedPointer::<u128>::ALIGNMENT, 8);
+            assert_eq!(TaggedPointer::<u128>::NUM_BITS, 3);
+
+            assert_eq!(
+                TaggedPointer::<u128>::POINTER_MASK,
+                0b11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111000usize
+            );
+        } else {
+            assert_eq!(TaggedPointer::<u128>::ALIGNMENT, 16);
+            assert_eq!(TaggedPointer::<u128>::NUM_BITS, 4);
+
+            assert_eq!(
+                TaggedPointer::<u128>::POINTER_MASK,
+                0b11111111_11111111_11111111_11111111_11111111_11111111_11111111_11110000usize
+            );
+        }
     }
 
     #[test]