Support setting alignment on globals

[Jules-Bertholet]

Pre-RFC: #[align] attribute

Summary Add an #[align] attribute to set the minimum alignment of struct and enum fields, statics, and local variables. Motivation Bindings to C and C++ C and C++ provide an alignas modifier to set the alignment of specific struct fields. To represent such structures in Rust, bindgen is sometimes forced to add explicit padding fields: // C code #include <stdint.h> struct foo { uint8_t x; _Alignas(128) uint8_t y; uint8_t z; }; // Rust bindings generated by `bindgen` pub struct foo …