first step toward cgo tool.
can extract import "C" doc comment
and all references to C package.
; cgo gmp.go | sort
#include <gmp.h>
gmp.go:197:4: mpz_t as type
gmp.go:205:2: mpz_init as call
gmp.go:206:2: mpz_set as call
gmp.go:221:2: mpz_init as call
gmp.go:227:7: size_t as call
gmp.go:228:2: mpz_export as call
gmp.go:235:13: mpz_sizeinbase as call
gmp.go:241:2: mpz_set as call
gmp.go:252:3: mpz_import as call
gmp.go:261:2: mpz_set_si as call
gmp.go:273:5: mpz_set_str as call
gmp.go:282:9: mpz_get_str as call
gmp.go:287:3: mpz_clear as call
gmp.go:302:2: mpz_add as call
gmp.go:311:2: mpz_sub as call
gmp.go:320:2: mpz_mul as call
gmp.go:329:2: mpz_tdiv_q as call
gmp.go:339:2: mpz_tdiv_r as call
gmp.go:348:2: mpz_mul_2exp as call
gmp.go:356:2: mpz_div_2exp as call
gmp.go:367:3: mpz_pow as call
gmp.go:369:3: mpz_powm as call
gmp.go:378:2: mpz_neg as call
gmp.go:386:2: mpz_abs as call
gmp.go:404:9: mpz_cmp as call
gmp.go:413:2: mpz_tdiv_qr as call
gmp.go:426:2: mpz_gcdext as call
;