Internal improvements: ASSERT_EQUAL/DEBUG_STRING macros, new JSON parser, better GMP<->MPFR conversion

[vasdommes]

• New ASSERT_EQUAL() and DEBUG_STRING() macros:

int x = 1;
int y = 2;
ASSERT_EQUAL(x,y) // Assertion 'x == y' failed: x='1' x='2'
auto s = DEBUG_STRING(x+x) // s = "x+x='2' "

• new JSON parser used for parsing block_data_XXX.json in SDPB and for functions.json and points.json in outer_limits.
• Use mpfr2mpf.h functions instead of stringstream for converting between GMP and MPFR bigfloats #172