Call-site macro

Came up [on discourse](https://discourse.julialang.org/t/ann-alloccheck-jl-static-code-analysis-to-prove-allocation-free-behavior/106414/8?u=carstenbauer).

Would be nice to have a macro that is similar to `@code_*` and others and that can be used like

```julia
@check_allocs multiply(rand(10,10), rand(10,10))
```

rather than having to augment the method definition (and run the allocation check on every call). I assume this is straightforward given that there already is the `check_allocs` function that works on the function signature alone.

Start by reading the existing check_allocs function, which the issue says already accepts a function signature, and compare it with the @check_allocs call-site example. The work is complete when the macro accepts an expression such as multiply(rand(10,10), rand(10,10)) and performs the allocation check without requiring a method definition change.

Written by the indexing model from the issue text.