Move allocation tracking into lib crate

We have several test cases defined in the `cli` crate that depend on the
`lib` crate's `allocation-tracking` feature.  The implementation of the
actual allocation tracker used to live in the `cli` crate, close to the
test cases that use it.  The `allocation-tracking` feature in the `lib`
crate was just used to tell the tree-sitter implementation to expect
that the allocation tracker exists, and to use it.

That pattern meant that we had a circular dependency: `cli` depends on
`lib`, but `lib` required some code that was implemented in `cli`.
That, in turn, caused linker errors — but only when compiling in certain
configurations! [1]

This patch moves all of the allocation tracking implementation into the
`lib` crate, gated on the existing `allocation-tracking` feature, which
fixes the circular dependency.

Note that this patch does **not** fix the fact that feature unification
causes the `lib` crate to be built with the `allocation-tracking`
feature enabled, even though it's not a default.  Fixing that depends on
the forthcoming version 2 feature resolver [2], or using the `dev_dep`
workaround [3] in the meantime.

[1] https://github.com/tree-sitter/tree-sitter/issues/919
[2] https://doc.rust-lang.org/nightly/cargo/reference/features.html#feature-resolver-version-2
[3] https://github.com/tree-sitter/tree-sitter/issues/919#issuecomment-777107086

Cargo.lock

@@ -836,7 +836,9 @@ name = "tree-sitter"
 version = "0.17.1"
 dependencies = [
  "cc",
+ "lazy_static",
  "regex",
+ "spin",
 ]
 
 [[package]]
@@ -862,7 +864,6 @@ dependencies = [
  "serde_derive",
  "serde_json",
  "smallbitvec",
- "spin",
  "tempfile",
  "tiny_http",
  "tree-sitter",