Add CLI option to reserve a fixed memory region at the start of the Solidity free-memory area

[PavelKopyl]

Feature Request

Summary
Add a new command-line option (e.g. --reserve-heap-start ) that carves out a reserved memory region beginning at offset 128 (the standard Solidity free-memory base). All other dynamic allocations,
the EVM stack spill area and any memory allocated by the compiled contract via the free memory pointer, must be placed after this reserved region.

Rationale

Some use cases require a fixed, compiler-untouched memory window at a well-known address immediately above the Solidity scratch/zero-slot area. For example, this covers custom caching mechanisms managed within inline assembly statements.
Currently there is no way to instruct solx to leave this area alone: the free memory pointer starts at 128 and both the spill area and contract allocations begin there immediately.

[hedgar2017]

@PavelKopyl we need to do this per-compilation-unit, don't we? In this case it's too heavy for the CLI, and can only be expressed in standard JSON input.

[PavelKopyl]

@PavelKopyl we need to do this per-compilation-unit, don't we? In this case it's too heavy for the CLI, and can only be expressed in standard JSON input.

OK, I think JSON will be enough for real use cases.