Use s-forms for op's like `len`, to mitigate stack copies, using consts

[phunanon]

Currently very innocuous op's like len, nth, type, not have their arguments duplicated to the stack before selecting only a small amount of data from them. A new special form for certain op's could ensure parameters and variable bindings are instead served as constants where appropriate. Even sect could benefit from this, with major rewriting.

[phunanon]

file-w and file-a would also benefit when it comes to referencing blobs. However, right now cast always copies by value, so I'll open another issue to have it only do so if the target IType requires zero'd out memory.

[phunanon]

It's too complicated to rewrite any op's leaning on ChVM::burstItem, such as sect.

[phunanon]

In a compiler (only a scrubber at this point in time) run this method reduced 25B average, not yet including the cast change. So far, even on Arduino, this memcpy avoidance is bringing trace performance improvements though.

[phunanon]

The concept of consts has been entirely removed with dbe523d with great optimisation benefits. This issue is no longer relevant.

[phunanon]

Surely there must be a way... maybe dedicated internal operations which do len et al., because len just needs to read two bytes, type doesn't even read the data itself, and not just needs to check truthiness!