Unnecessary avo dependency

[mmcloughlin]

The sha1cd module has an unnecessary dependency on avo, since avo is only required for code-generation. It seems this has caused problems for sha1cd users:

sourcegraph/zoekt#893
mmcloughlin/avo#455

A common pattern for avo code generators is to use a separate module. The Go standard library calls these _asm, for example:

https://github.com/golang/go/blob/80bf7d83edbb48e2411d755f0636565467bb5a56/src/crypto/md5/_asm/go.mod

Other projects call it asm or avo.

[mmcloughlin]

PR #149 shows an example of how this could be fixed. It gets the job done but might need some additional cleanup, for example the shared module perhaps does not make sense with this arrangement.