Generate combinational tests for linear memory functions

[jayz22]

Generate linear memory tests

What

Resolves #1144

This PR introduces new macro code synth_linear_memory_tests.rs that generates tests (tests/linear_memory.rs) for linear memory host functions.
There are three different types of test, depending on the form of input argument combinations (a host function may have multiple):
a). "lm_pos + len (bytes)" where either the initial lm_pos or the range can be out of bound. Involved host functions:

• "bytes_new_from_linear_memory"
• "string_new_from_linear_memory"
• "symbol_new_from_linear_memory"
• "bytes_copy_from_linear_memory"
• "bytes_copy_to_linear_memory"
• "string_copy_to_linear_memory"
• "symbol_copy_to_linear_memory"

b). "val_pos + len (number of Vals)", where either the initial val_pos or the range can go out of bound. Here Val must also be 8 bytes forming a valid env::Val. Involved host functions:

• "map_new_from_linear_memory"
• "vec_new_from_linear_memory"
• "map_unpack_to_linear_memory"
• "vec_unpack_to_linear_memory"

c) An array of keys where each key is a 8-byte slice consist of 4-byte ptr + 4-byte len, where either the ptr or len can go out of range. Involved host functions:

• "map_new_from_linear_memory"
• "symbol_index_in_linear_memory"
• "map_unpack_to_linear_memory"

It also involves generating a wasm module for each host function, setting up sections of linear memory, preload it with some values. Please refer to the big comment in the file for details.

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

[TODO or N/A]

[graydon]

In general I'm only so-so comfortable with this code as-written:

• The code imperatively fills linear memory rather than using a static data section in each wasm, which makes the tests run slower than they need to and avoids exercising the data-section paths (it's also confusing that the code talks about sections while not actually using sections; it seems to just be using the term "section" as a synonym for "region" or "extent" or something, but "section" has a specific meaning in wasm modules)
• The code probably doesn't have to be macro-generated; I can see starting there from the perspective of giving yourself maximum freedom to factor the work / maximum generated-tests-per-unit-of-effort but the effect is to make the tests harder to understand and maintain long-term (this is always the risk with macros or any kind of metaprograms)

That said: we're in a rush and these tests do seem to exercise the surface area we want, so I'm going to merge now. I'll file a followup bug to maybe revisit these tests in the future and simplify them.