Reuse memory slots

[iamdefinitelyahuman]

What I did

Allow for releasing and reuse of memory slots.

How I did it

• Within the memory allocator, add a method release_memory that accepts an offset and length. Released memory is tracked in a a list using FreeMemory objects. These objects are essentially 2 item lists, but I think the special class makes the code more readable.
• Calls to increase_memory check for available slots in released memory, and if found they are reused instead of expanding memory.
• Upon exiting the make_blockscope context manager, all memory variables that are no longer accessible are released.
• Added a new context manager internal_memory_scope. It works identically to make_blockscope. I added it to differentiate between an actual user-defined scope (for loop, if branches) and an internal scope (event generation, assertions). I applied it to several areas where we generate temporary memory variables.

How to verify it

Run the tests. I haven't added any specific cases around this, but it should factor into the majority of the test suite. I verified the behavior by compiling and running the tests at https://github.com/curvefi/curve-contract and everything works.

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