Replace std::bind with lambdas in eetf/ei.hpp

[stephenberry]

Summary

Replaces every std::bind call site in include/glaze/eetf/ei.hpp with an equivalent lambda. This is a pure refactor with no behavioral change.

The EETF decode/encode helpers wrapped the C ei_* functions with std::bind(ei_func, _1, _2, trailing_args...) and passed the result to detail::decode_impl / detail::encode_impl. Each is a textbook trailing-argument bind, which a lambda expresses more clearly and compiles better.

Why lambdas here

• Inlining. std::bind returns an opaque call wrapper routed through std::invoke; compilers inline through it far less reliably than a lambda, especially in Debug. These helpers are all GLZ_ALWAYS_INLINE hot paths, so the codegen difference is real.
• No decay-copies. std::bind decay-copies each bound argument into the wrapper; a [&] capture avoids that.
• Clarity + a stricter signature. Drops the _1/_2 placeholders and the per-function using namespace std::placeholders;. The fixed-signature lambda also makes the ei_* call's argument list explicit and would be a compile error on a signature mismatch, where bind would silently swallow it.
• It's the standard guidance (Effective Modern C++ Item 34, "Prefer lambdas to std::bind").

Scope

• 23 std::bind sites converted (10 decode, 13 encode); 8 now-unused using namespace std::placeholders; lines removed.
• The lambda parameter types follow each helper's invocation contract: decode_impl invokes the functor once as func(const char*, int*); encode_impl invokes it twice (a nullptr sizing pass then the real buffer) as func(char*, int*). The bound trailing arguments are passed through unchanged.
• No include change: ei.hpp did not directly include <functional>.

Verification

• Compile-checked locally (clang++ -std=c++23 -fsyntax-only -Wall -Wextra) via a harness that instantiates both the decode path (read_term) and the encode path (write_term) over a struct exercising bool / int / double / long long / unsigned long long / string / atom / vector / map (map header + encode_atom_len keys) / binary / tuple. Clean.
• Full build + tests run in CI via the clang-linux-erlang and gcc-erlang jobs, which link against erl_interface and exercise these paths.