feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/cloudflare/circl/math/fp448/fp_noasm.go

@@ -0,0 +1,12 @@
+//go:build !amd64 || purego
+// +build !amd64 purego
+
+package fp448
+
+func cmov(x, y *Elt, n uint)  { cmovGeneric(x, y, n) }
+func cswap(x, y *Elt, n uint) { cswapGeneric(x, y, n) }
+func add(z, x, y *Elt)        { addGeneric(z, x, y) }
+func sub(z, x, y *Elt)        { subGeneric(z, x, y) }
+func addsub(x, y *Elt)        { addsubGeneric(x, y) }
+func mul(z, x, y *Elt)        { mulGeneric(z, x, y) }
+func sqr(z, x *Elt)           { sqrGeneric(z, x) }