Allow 'inline' keyword in declarations

[jyn514]

For now, it should just be a no-op.

static inline void f(); 
<stdin>:1:8: error: expected variable name or '(', got 'inline'

[jyn514]

Steps to get started:

1. Optionally, parse inline as a Qualifier in declaration_specifier
2. Give an error for inline int a in declarator
3. Write some test cases in tests/runner-tests/decls/

[jyn514]

Current behavior:

$ RUST_BACKTRACE=1 cargo run
    Finished dev [unoptimized] target(s) in 0.54s
     Running `target/debug/rcc`
int main(restrict int *p);
thread 'main' panicked at 'unrecognized declaration specifier restrict', src/parse/decl.rs:1486:25
stack backtrace:
  13: rcc::parse::decl::<impl rcc::parse::Parser<I>>::declaration_specifier::{{closure}}
  14: core::result::Result<T,E>::unwrap_or_else
  15: rcc::parse::decl::<impl rcc::parse::Parser<I>>::declaration_specifier
  16: rcc::parse::decl::<impl rcc::parse::Parser<I>>::declaration_specifiers
  17: rcc::parse::decl::<impl rcc::parse::Parser<I>>::parameter_type_list
  18: rcc::parse::decl::<impl rcc::parse::Parser<I>>::postfix_type
  19: rcc::parse::decl::<impl rcc::parse::Parser<I>>::direct_declarator
  20: rcc::parse::decl::<impl rcc::parse::Parser<I>>::declarator
  21: rcc::parse::decl::<impl rcc::parse::Parser<I>>::declaration
  22: <rcc::parse::Parser<I> as core::iter::traits::iterator::Iterator>::next::{{closure}}
  23: core::option::Option<T>::or_else
  24: <rcc::parse::Parser<I> as core::iter::traits::iterator::Iterator>::next
  25: rcc::compile
  26: rcc::real_main
  27: rcc::main