flutterdec v0.1.0-alpha.4

First release with binaries built outside the Nix dev shell: Linux is a
static-pie musl build, macOS links system libiconv. v0.1.0-alpha.3 was
withdrawn because its artifacts could not start without Nix.

Object pool references now resolve in the real index space, fixing
pseudocode that rendered string literals the binary never referenced.
Adds the r2flutter adapter backend and a self-documenting --help.