headers: split public and private API headers

[jow-]

Split the ucode headers into an installed public API and a private include/ucode/internal/ tree that is never installed, so downstream codebases no longer pick up implementation details -- in particular the bare 'unused' macro that clashed with downstream definitions.

Public (installed) headers:
ucode.h umbrella
types.h value types + public value ABI + struct uc_vm
vm.h VM API (ISA moved out)
compiler.h uc_compile() + parse configuration
source.h, program.h, lib.h, module.h, util.h, platform.h
util.h deprecated shim (kept for compatibility)

Private (include/ucode/internal/, not installed):
types.h, vm.h, compiler.h, source.h, program.h, chunk.h, vallist.h,
lexer.h, platform.h, util.h, util-macros.h

Key points:

• The bare 'unused'/'localfunc' macros are isolated in internal/util-macros.h; the installed util.h no longer defines them.
• struct uc_vm stays public: downstream (rpcd, uhttpd, uwsd) embeds uc_vm_t by value and calls uc_vm_init()/uc_vm_free(), so it cannot be opaque. uc_source_t/uc_program_t/uc_function_t are opaque forward decls; their full layouts (plus chunk, thread-context, object-iterator) live in internal/types.h.
• The bytecode ISA (_insns, uc_vm_insn_t, I* opcodes) moves to internal/vm.h; compiler state to internal/compiler.h.
• compile.h is merged into compiler.h (the former is removed); the public compilation API lives in a single header.
• lexer.h moves to internal/ (no downstream users).
• platform.h is split: the public half keeps UC_SYSTEM_SIGNAL_COUNT and uc_system_signal_names[]; byte-order helpers, execvpe(), pipe2(), sigtimedwait() and the environ shim move to internal/platform.h.
• UCODE_BYTECODE_VERSION moves to program.h (it describes the serialized bytecode, not the VM).
• uc_source_get/put and uc_program_get/put become real functions (the types are now opaque); the __hidden source/program internals move to internal/source.h and internal/program.h.
• chunk.h and vallist.h move to internal/ (unused downstream).
• lib.h uc_fn_this reads vm->callframes directly (struct uc_vm is public).
• UC_GC_ENABLED / UC_GC_DEFAULT_INTERVAL introduced; bare GC_* kept as deprecated aliases.
• Internal headers include their own public counterpart but no other public header; .c files include the internal header only. Redundant includes are dropped from every .c file (verified by compile-testing each removal); conditional (#ifdef) includes are preserved.

Fixes: #429
Supersedes: #430