Switch to slab allocators, add allocator API

[MasonProtter]

This is yet another breaking release, but I'm hoping this is the last one we'll need before 1.0, but no promises.

• With this PR, the default allocator is now something known as a slab allocator SlabBuffer. This comes with a very slight performance hit relative to AllocBuffer, but the advantage is that it scales very well from handling small allocations all the way up to handling very large allocations. It will only run out of memory when your computer runs out of memory, but it also won't hog memory that's not in use. It is also be much faster to construct than the old default AllocBuffer. Closes #9

• AllocBuffer still exists, but now defaults to 128kb of storage instead of 1/8th of your computer's physical memory. This allocator is very slightly faster than the slab allocator, but will error if it runs out of memory. It also is more flexible in the kinds of types it can wrap to use as underlying storage.

• There is now an API for hooking user-defined allocators into the @no_escape and @alloc machinery.

  • alloc(::Type{T}, buffer, dims...) is now alloc!(buffer, ::Type{T}, dims...)

• alloc_nothrow and @alloc_nothrow have been removed. People who need this can instead create custom no-throw buffer types. Closes https://github.com/MasonProtter/Bumper.jl/issues/3

Here's a preview of the new revamped README: https://github.com/MasonProtter/Bumper.jl/blob/slab-overhaul/README.md and a new page for docstrings: https://github.com/MasonProtter/Bumper.jl/blob/slab-overhaul/Docstrings.md

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4a6a1cd) 86.17% compared to head (1911aea) 98.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #24 +/- ## =========================================== + Coverage 86.17% 98.63% +12.46% =========================================== Files 2 4 +2 Lines 94 147 +53 =========================================== + Hits 81 145 +64 + Misses 13 2 -11 ``` | [Files](https://app.codecov.io/gh/MasonProtter/Bumper.jl/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mason+Protter) | Coverage Δ | | |---|---|---| | [src/AllocBuffer.jl](https://app.codecov.io/gh/MasonProtter/Bumper.jl/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mason+Protter#diff-c3JjL0FsbG9jQnVmZmVyLmps) | `100.00% <100.00%> (ø)` | | | [src/SlabBuffer.jl](https://app.codecov.io/gh/MasonProtter/Bumper.jl/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mason+Protter#diff-c3JjL1NsYWJCdWZmZXIuamw=) | `100.00% <100.00%> (ø)` | | | [src/internals.jl](https://app.codecov.io/gh/MasonProtter/Bumper.jl/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mason+Protter#diff-c3JjL2ludGVybmFscy5qbA==) | `100.00% <100.00%> (+13.33%)` | :arrow_up: | | [src/Bumper.jl](https://app.codecov.io/gh/MasonProtter/Bumper.jl/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Mason+Protter#diff-c3JjL0J1bXBlci5qbA==) | `89.47% <88.23%> (+14.47%)` | :arrow_up: |

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