[Breaking] Replace alloc with @alloc

[MasonProtter]

This is safer, and also in some situations faster. Before you'd write

function f(x::Vector{Int})
    @no_escape begin # This called default_buffer()
        y = alloc(Int, length(x)) # This also called default_buffer()
        y .= x .+ 1
        sum(y)
    end
end

and now you write

function f(x::Vector{Int})
    @no_escape begin  # Only this calls default_buffer()
        y = @alloc(Int, length(x)) # this uses whatever buffer `@no_escape` uses.
        y .= x .+ 1
        sum(y)
    end
end

@alloc no longer takes an optional buffer argument, it is always associated with the @no_escape block that encloses it, and it cannot be used outside of a @no_escape block.

This makes @alloc a bit more like alloca in some senses. I think it was a mis-feature to allow alloc to be used lexically outside of a @no_escape block.

I also spruced up the README a little and added docstrings to the API functions. Preview of the new README here: https://github.com/MasonProtter/Bumper.jl/blob/alloc-macro/README.org

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (8126213) 63.15% compared to head (4186def) 85.86%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #20       +/-   ##
===========================================
+ Coverage   63.15%   85.86%   +22.71%     
===========================================
  Files           1        2        +1     
  Lines          76       92       +16     
===========================================
+ Hits           48       79       +31     
+ Misses         28       13       -15     

Files	Coverage Δ
src/Bumper.jl	75.00% <50.00%> (+11.84%)	⬆️
src/internals.jl	86.36% <86.36%> (ø)

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