gccrs: libproc_macro: Add member functions to Literal

gccrs: libproc_macro: Add member functions to Literal

Add some member functions to the Literal structure as well as named
constructors.

libgrust/ChangeLog:

	* libproc_macro/literal.cc (Literal__string): Add call
	to named constructor.
	(Literal__byte_string): Likewise.
	(Literal::make_unsigned): Add function.
	(Literal::make_signed): Add function.
	(Literal::clone): Likewise.
	(Literal::make_u8): Likewise.
	(Literal::make_u16): Likewise.
	(Literal::make_u32): Likewise.
	(Literal::make_u64): Likewise.
	(Literal::make_i8): Likewise.
	(Literal::make_i16): Likewise.
	(Literal::make_i32): Likewise.
	(Literal::make_i64): Likewise.
	(Literal::make_string): Likewise.
	(Literal::make_byte_string): Likewise.
	(Literal::make_f32): Likewise.
	(Literal::make_f64): Likewise.
	(make_char): Likewise.
	(Literal::make_char): Likewise.
	(make_usize): Likewise.
	(Literal::make_usize): Likewise.
	(make_isize): Likewise.
	(Literal::make_isize): Likewise.
	* libproc_macro/literal.h: Add prototypes.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>