Start prototyping JIT compiler

[playXE]

Since Starlight aims to be a fast JS engine JIT compiler is necessary but there are still unresolved questions:

• What JIT technique to use? Tracing JIT or Method JIT?
• What backend to use? Cranelift vs LLVM vs MIR vs our own (probably rewrite WebKit's macro assembler)
• How many tiers if we will use Method based compilation?
• How to profile everything and how do we emit efficient code for method JITed code?
• If we will use tracing JIT then how to record bytecode trace? Do we need an entire separate interpreter or we could just copy-paste the existing one?

[playXE]

I decided that Starlight will use a tiered method-based JIT compilation. There will be single-pass JIT compiler that simply emits unoptimized machine code and second tier will utilize all the profiling information to emit optimized code.