Rename include folder & rename symbols to npymath_*

- Rename include folder so that `#include <libnpymath/npymath.h>`
  has to be used.
- Rename npymath_* symbols, so that there are no conflicts with old
  symbols in NumPy

meson.options

@@ -2,7 +2,7 @@ option('numpy_build', type: 'boolean', value: false,
        description: 'Whether libnpymath is getting built as part of numpy')
 option('install_lib', type: 'boolean', value: true,
        description: 'Whether to install the libnpymath library')
-option('headers_dir', type: 'string', value: 'libnpymath',
+option('headers_dir', type: 'string', value: '',
        description: 'Installation directory for the libnpymath headers')
 option('lib_dir', type: 'string', value: 'libnpymath',
        description: 'Installation directory for the libnpymath library')

src/arm64_exports.c

@@ -11,19 +11,19 @@
  * This file is actually compiled as part of the main module.
  */
 
-double npy_asinh(double x) {
+double npymath_asinh(double x) {
     return asinh(x);
 }
 
-double npy_copysign(double y, double x) {
+double npymath_copysign(double y, double x) {
     return copysign(y, x);
 }
 
-double npy_log1p(double x) {
+double npymath_log1p(double x) {
     return log1p(x);
 }
 
-double npy_nextafter(double x, double y) {
+double npymath_nextafter(double x, double y) {
     return nextafter(x, y);
 }