be-py, be-rust: name a library with a digit the way python and cargo do

[notactuallytreyanastasio]

A library named f64str or radix-36 builds on py and rust, then
fails to run, because each backend refers to it by a name nothing
defines:

import f64_str as module
ModuleNotFoundError: No module named 'f64_str'

error[E0433]: cannot find module or crate `radix36` in this scope
2 |     radix36::init(None).unwrap().run_all_blocking();

Both names came from IdentStyle.Dash.convertTo(IdentStyle.Snake, ..).
That conversion re-splits the name into words, and it puts _ after a
run of digits but never before one, so f64str becomes f64_str and
radix-36 becomes radix36. It is the right tool for turning a
Temper identifier into a Python or Rust one, and the wrong one here,
where the name has to match what another tool already made of it.

py writes the package to a directory named by toModuleFileName
(f64str, radix_36), and imports of it from other libraries use
that too; only the PyLibraryName metadata, which top.py and the
REPL import, used IdentStyle. pyDirToLibraryName now uses
toModuleFileName.

cargo names a package's library crate by replacing each - with _.
PackageNaming.crateName now does exactly that. It is used for
main.rs and for paths into other crates, so a library that imports
from radix-36 failed on rust the same way, with radix36::, and now
prints its result too. Module paths inside a crate still use
dashToSnake; they are declared and referred to by the same function.

Names with no digit come out as before: both conversions give
hello_world for hello-world.

PyIdentifierTest.libraryNameIsItsPackageDirectory and
RustNamesTest.crateNameIsCargos fail before this change on f64str
(f64_str) and pass after. temper run of f64str, radix-36, and a
library importing from radix-36, prints the expected lines on py and
rust.

Fixes #610.

Reproduction: 57-library-name-with-digit/ in temper-issue-repros.

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