Update mathematical functions (#675)

Document the existing `ROUND`, `CEIL`, and `FLOOR` functions
Also update the trig function docs for searchability

Implement `POW` and `LOG`
Addresses part of #675

Implement ** for integer exponents
** has higher precedence than -, like Python, so -3**4 == -(3**4) == 81

test/asm/math.asm

@@ -0,0 +1,40 @@
+X equ 0
+
+test: MACRO
+; Test RGBASM
+v equs "X +"
+	static_assert \#
+	purge v
+; Test RGBLINK
+v equs "Y +"
+	assert \#
+	purge v
+ENDM
+
+	test (v 2)*(v 10)**(v 2)*(v 2) == (v 400)
+	test -(v 3)**(v 4) == v -81
+
+	assert DIV(5.0, 2.0) == 2.5
+	assert DIV(-5.0, 2.0) == -2.5
+	assert DIV(-5.0, 0.0) == $8000_0000
+
+	assert MUL(10.0, 0.5) == 5.0
+	assert MUL(10.0, 0.0) == 0.0
+
+	assert POW(10.0, 2.0) == 100.0
+	assert POW(100.0, 0.5) == 10.0
+
+	assert LOG(100.0, 10.0) == 2.0
+	assert LOG(256.0, 2.0) == 8.0
+
+	assert ROUND(1.5) == 2.0
+	assert ROUND(-1.5) == -2.0
+
+	assert CEIL(1.5) == 2.0
+	assert CEIL(-1.5) == -1.0
+
+	assert FLOOR(1.5) == 1.0
+	assert FLOOR(-1.5) == -2.0
+
+SECTION "Y", ROM0
+Y::