pyo3-build-config: Replace TargetInfo with target_lexicon::Triple

Add a new public crate function `cross_compile_from_to()` using `Triple`
arguments instead of plain strings in `cross_compile()`.

The downsides:

1. Build times: A simple PyO3 extension module build time increased (on average)
   from 15.05s to 15.25s (+0.2s) because of the new target-lexicon dependency
   and its build script.

2. The public crate API function `cross_compile()` may become somewhat flaky
   because of the impedance mismatch between Cargo and `target-lexicon`
   target triple interpretations.

pyo3-build-config/Cargo.toml

@@ -12,6 +12,10 @@ edition = "2018"
 
 [dependencies]
 once_cell = "1"
+target-lexicon = "0.12"
+
+[build-dependencies]
+target-lexicon = "0.12"
 
 [features]
 default = []