push! doesn't work on BBs

Not sure if this is intended but https://github.com/MikeInnes/IRTools.jl/blob/bacecb8a71eb0026963021d33e4ba16bfc7211da/src/ir/ir.jl#L630 doesn't work on BBs, as illustrated:
```
function block_transform(ir)
ir = copy(ir)
for (i, bb) in enumerate(ir.blocks)
for (j, stmt) in enumerate(bb.stmts)
stmt.expr.head == :call &&
stmt.expr.args[1] isa GlobalRef &&
stmt.expr.args[1].name == :rand &&
begin
new = argument!(ir)
new_stmt = IRTools.Inner.Statement(
Expr(:call,
GlobalRef(stmt.expr.args[1].mod, :logpdf),
stmt.expr.args[2:length(stmt.expr.args)]...,
new),
stmt.type,
stmt.line)
v = push!(bb, new_stmt)
end
end
end
return renumber(ir)
end
```
which throws
```
ERROR: LoadError: MethodError: no method matching push!(::IRTools.Inner.BasicBlock, ::IRTools.Inner.Statement)
```
because there's no inheritance
https://github.com/MikeInnes/IRTools.jl/blob/bacecb8a71eb0026963021d33e4ba16bfc7211da/src/ir/ir.jl#L129-L134

It looks like it's easy to implement this i.e. in the `/src` for `push!`:
```
push!(BasicBlock(b).stmts, x)
push!(b.ir.defs, (b.id, length(BasicBlock(b).stmts)))
return Variable(length(b.ir.defs))
```

I think this issue is about a more general confusion of the difference between `Block` and `BasicBlock` (i.e. the documentation uses block to possibly refer to both?)